Horribly complicated. (Warning - contains maths)

;D Just one of those cleverly worded things that leads you in completely the wrong direction.

Think of it like this:

The meal originally came to £30.
Each man paid £10 towards the meal.
The waiter returns with £5 change, from which he takes £2 for a tip (ok so they give it to him later, but forget that and cut to the chase, makes it easier to see what's going on.)
What's left from the £5 refund is the £3 that they divide between them.

Add their £3, the waiter's £2 and the £25 bill and you se that it still comes to £30.

See, all a question of wording. No money ever gets lost.
It is a good one though, and can certainly keep people going for a while ;D ;D