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.
In some situations, it's convenient to autofocus then switch to manual focus so the focus doesn't change (timelapse, prefocussing for a remote-triggered shot etc.). This wouldn't make sense if you had to turn the camera off in between.