Yep, I think life is indeed that kind and that your friend is misinformed. She may have said that because where she works has that policy but, in the absence of anything to the contrary (as I said, in a handbook, contract, by practice etc.) then your employer can't take back the money. So you can do exactly as you suggest - come back for a few weeks and then hand in your notice. From a relationship point of view, it may be better to be upfront about this - if they have employed someone in your absence, they may want to just keep that person on and, if you come back for a few weeks and say nothing, they may have missed that chance to keep that other person on. Their noses will probably be out of joint if you come back, say nothing and then just leave or leave fairly shortly after, but them's the breaks. It's totally up to you.
Or suggest that you go back part-time (if you would be happy with that yourself) - then your employer can make an informed decision on that (you, part-time or someone else full time) and they would feel a little more in control of the decision - and if they say they can't do part-time then everyone goes their separate ways with no harm, no foul.
Sprite