That's a big no-no. Gambling with another person's money is a great way to lose that person's trust and friendship.
I'll second that as much i love gambling it's with my own never with any one else money, simple reason one I can't be answerable to anyone, plus one can't be at mercy of others. I would like to have that friend in my life, money comes and goes but friend are always. I think people should not do this, as it will destroy their credibility in the other persons eye.
Playing gambling with someones else money seems to be an illogical thing to me because when someone would give you your money even when he knows that the money you are taking form him is not to meet his need, its just to meet his love of life and his aim. Also, if one gambles with that money, his reputation in the eyes of other would be nothing.