I've never personally been scammed, I keep out of Paypal trading, and I'm generally fairly scrutinizing. I do know however that people come up with really strange and odd methods of scamming.
As a general rule, as a tip to your friend, never make 2 step trades for gift wrap. Once an item is wrapped, you can offer the now wrapped item in the trade window. In any gift wrap requiring stage, the best policy is often to trade your offer + 1 key and a few rec, for the already gift wrapped item, preventing any possible scamming.

I was scammed in a gift wrap scam once, only lost 2 ref though so it wasent that bad, sorry for your friend, but it is pretty pathetic that he scammed for a measly 2 keys.

some dude came on a trade server once and was saying "I paint hats any color you want" and some stupid ass gave him his hat. that shit was funny! i mean seriously if you are that stupid to give someone your hat with no collateral you shouldnt be allowed to trade in TF2. lawlz

Read some of this to save you getting scammed in future: http://www.tf2pricelist.com/tf2-trade-secrets.html

I was scammed out of a few lower class weapons I had doubles of that way. The guy who scammed me said he would gift a deus glasses to me. Never got them first or last.

one time someone tried scamming me out of hats by putting up an automizer and saying it was worth 2 ref. i was smart enough not to do it but it was a funny fail XD