If your friend is actually your friend and also treated the mod right, then possibly.
I always say (personally) I wouldn't buy a mod from a friend, it's kind of like loaning money to a friend. I don't do it. If they need help and I want to give them $$ I will, but I always explain to myself that even if it's a "loan" it's really a "gift" so I don't get annoyed if I don't get paid back.
My approach with a mod is, if I buy it, I better be prepared for it to die (even though it might not) and if I can live with the price of the mod then fine, but I don't necessarily expect it not to die, and I have to be okay with it if it does.
So basically.... Yes, it's OK if you want to, but you have no guarantees of the state of the mod, really.
Anna