Unless you can come up with a plausible reason why you would be buying her son a present that isn't the truth, I'd steer clear of handing it to her.  You will wind up getting burned. 

 

I fully understand your reasoning, but you have to think of both her pride and his. 

 

A secret Santa type delivery would be your best bet - dropped on the doorstep for him to find after school or sent signature required by courier to him specifically.  It's just a matter of ensuring it gets straight to his hands, not hers.