I don't know, I'd side with the manchild in this because the dipshit shouldn't have thought it was okay to just take something and expect nothing to happen. It also says something about the other parents poor parenting that he's raised someone, who sounds passed the age where it's a dumb kid being dumb, that thinks it's okay to just take whatever and even worse he seems to be defending that behaviour. Sure, it might be some dumb toy in this case, but having his poor behaviour not disciplined gives me the feel that it's only going to continue or escalate.
I've no idea how expensive the specific figure is as I don't care to check, but they can get pretty up there and even more so for exclusives, not just the case of someone getting upset the kid stole something he could pick up for $20 at walmart. It's more the issue that something expensive has just been stolen by an invited guest, replace that figure with anything expensive of your own and you'd probably feel similar. Just because it's a toy doesn't change this, if I spent 100+ on anything I'd be pissed at someone just up and taking it because they felt like it.