Question: I have a Canon 700d with 2 kit lenses, both having auto-focus feature. I read a book "Canon EOS Rebel T5i/700D For Dummies", which warns against switching auto-focus on/off while the camera is on. According to it, we should first turn off the camera before turning auto-focus on/off. That's quite cumbersome obviously.

A few days ago, a friend of mine who is professional photographer noticed me doing this, and told me that doesn't matter and I need not turn off my camera while switching auto-focus on/off. I am confused if this might harm my camera or lens in any way?

Answer: 
The warning you read applies to Live View shooting. If you look at the T5I manual it states on page 156 under the Continuous AF section , "...During Continuous AF, turn off the power before you set the len's focus mode switch to MF". I usually only use Continuous AF on my T4I so if I do switch to manual I don't usually remember to do this, and I have never had an issue. With that stated, following the camera manufacturer's advice is the safe way to go.
The risky AF switching may be:

* Interferrence between in-body motor and in-lens rings. it is simillar to changing gears in car without clutch pressed. All EF lenses have electronic interface only, so this is not your case.
* Switching off the motor under load. When switching the switch, peak currents may be delivered to the motor or the control unit causing damage. This may be your case but it is risky if
* you are switching the AF during autofocus routine;
* you are switching the AF during live view and contionuous AF.

If the AF motor is not engaged it is 100% safe to switch the AF on/off on EOS lens. Same is valid for IS or any feature - do not switch the power off under load!

I don't think it will immediately kill the lens, but it is unnecessarily rude. In emergency - The Perfect Moment is approaching and AF still cannot focus - or -I shooting video and it focus on wrong point - do it. But take it as a lesson not as a routine.