SUBREDDIT: r/relationships
TITLE: Girlfriend of over 2 years [16/F] has secret attraction to one of my [16/M] best friends.
POST: I know that reddit doesn't tend to take well to teenagers normally, so I'm expecting downvotes for that alone, but any feedback would be nice.

What's happening is this: I started dating my girlfriend over 2 years ago and over the 2+ years it's been absolutely great. 

Recently though, I've realized that she's been flirting with one of our mutual friends quite a bit and always seems to want to turn our normal 2 people dates into 3 person affairs. A quick chat with one of my friends confirms that she is attracted to him and has been for about 6 months now.

From what I'd gathered from asking around, she doesn't seem like she enjoys being attracted to this mutual friend, and she still acts the same around me as she has for the 2 years so I doubt there is any loss of attraction coming my way (i.e, still plenty of flirting, kissing, and spending time with each other).

I also don't think our mutual friend would do anything since he's generally pretty socially awkward, doesn't know the first thing about the opposite sex, and I would like to think that he wouldn't do anything to hurt one of his only real friends (me).

My girlfriend is currently on vacation and will be coming back mid of next week when I plan on confronting her about this. Mainly, I'm not sure how I should go about talking to her about this in such a way that it doesn't come off as me being a jackass of a boyfriend or make it seem like I want her to cut every time with this guy for the rest of her life.

My plan for right now is to ask if she does have any sort of feelings for him, and play it off like I managed to deduce she did (which isn't far from the truth, I was starting to get suspicious before I even found out for sure).

Any advice from you guys would be greatly appreciated!
TL;DR: 
Girlfriend of 2+ years has a thing for a mutual friend. Not sure how to confront her with my knowing when she comes back from vacation.