The simple answer is burn a human effigy in any bonfire.
It will temporarily prevent invasions.
gaming.stackexchange.com/users/13121/warsong
Disconnecting the console from the internet is the only sure way to prevent player invasions in Dark Souls 2.
That said, there are many conditions for both being able to invade and being susceptible to invasions, so your friend shouldn't have to worry too much.
In my 72 hours of playing the game, I've been invaded only 3 times, not counting the areas belonging to the PvP-covenants.
Those are entirely optional.
Like the comments above mention, you can minimize invasions while online for about an hour by burning a human effigy at a bonfire.
The item description for the human effigy mentions this effect:
A warm, soft, shadow-like effigy.
Use this item to reverse Hollowing.
It also weakens the links to other worlds, preventing invasions and most cooperation.
Peer closely at an effigy, and one begins to perceive a human form, but whose form it takes depends on the person looking.
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The Way of Blue supposedly summons available members of the Blue Sentinels covenant to help the Way of Blue player being invaded.
I don't believe it matters whether the Way of Blue player is hollow or not, but the Blue Sentinel must be human, wearing the ring and also not fighting a boss or near a boss gate.
With all these conditions, I'm not surprised people have seen few protectors these first few weeks after the game is out.
Only a portion of players elect to join the Blue Sentinels, and then they both have to sacrifice a ring slot and not lose their humanity to be able to help.
Since your comment mentions the original Dark Souls, I'll add that in that game you would never be invaded if hollow; only in human form.
So you could easily play through the whole game without disconnecting your console and never be invaded.
In my experience, invasions have resulted some of the most nerve-wrecking and adrenaline-filled battles in the Souls series.
They've also been some of the most memorable, so I recommend not trying to prevent them.
gaming.stackexchange.com/users/54348/Valdimar
You don't necessarily have to disconnect from the internet, just sign out from XBox
Live/PSN while playing.
I only do this when I am in the Rat-Bro areas (sorry guys, but I am on a mission and it isn't avoiding your trap and spell spam) mostly because I always seem to get pulled into their worlds at the most annoying times.
I guess you could just always sign out when invaded, but your sin will skyrocket (and it's annoying for legit pvp-ers).
After disconnecting too many times, the game will disconnect you from all multiplayer content for something like 2 real weeks (wouldn't know exactly, but I heard it happens to people with bad connections who get dropped by the client
/server too many times).
My personal favorite is to use the Seed of a Tree of Giants to turn enemies against the invader and/or make a break for the boss door if you're close enough).
gaming.stackexchange.com/users/73448/BloodReyvyn
Yup just start an Xbox live party and it will put you in offline mode.
Plus you still get to talk without worry of invasions
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Hope this isn't too late to convince your friend to play.
The only way to turn off invasions is to turn off all online play by disconnecting your computer/console from the internet.
Which unfortunately turns off a lot of the really cool aspects of this game.
That said, the Way of the Blue covenant is super-helpful for protecting against invasion.
I've had a Blue Sentinel pop in each time I've been invaded.
Even had a second one pop in once after the first one died.
If you're human (which you aren't always, but that can be easily fixed with a small soap stone), you can even call in some white phantoms/shades to help you.
gaming.stackexchange.com/users/75672/Reynard-Miri
How to stop invaders Take your console offline Sounds stupidly simple, but simply disconnecting your console from the internet will prevent any and all players from invading your game.
It’s an inelegant solution, but one that is guaranteed to work.
If you disconnect during an invasion, the game punishes you by being unable to summon or be summoned.
Using a Bone of Order resets this penalty.2.
Use a Seed of the Tree of Giants Seed of the Tree of Giants turns all enemies in the area hostile towards an invader.
As soon as one enters your game, use it, especially if you’re in a tough part of Drangleic.
Former enemies become essential allies in banishing invaders back to where they came from.
There is a tree in the Forest of Fallen Giants which, every time you’re invaded, has the chance of dropping one of these seeds, you can travel here and see if one has spawned, evening the odds a little upon the next invasion.
Use the Black Separation Crystal As soon as the sign pops up informing you of an invader entering your world, head into your inventory and use the Black Separation Crystal.
This item banishes invaders back to their own world, or if you’re an invader in a spot of bother, leave the world you’ve entered.
gaming.stackexchange.com/users/77308/user77308
My friend recently started playing Dark Souls 2...and the low-risk of invasion has immediately turned him off from the game.  I want to encourage him to give it a try, and I feel like turning off this online capability would help him enjoy the main story a lot more.   I know there's a convoluted way to do this for the original Dark Souls, but is there a way to do this in Dark Souls 2?
Is there any way to turn off invasions in Dark Souls 2?
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gaming.stackexchange.com/users/55285/Zibbobz
You can burn a human effigy in any bonfire will reduce invasions. Disconnecting the console from the Internet is the only way to prevent player invasions. While there are in game ways to deal with the invasion problem, most players tend to go offline.
You don't necessarily have to disconnect from the internet, just sign out from Xbox
The community suggests few in game ways to stop invasions. While few others propose to go offline.