Okay, he went with the player/possible GF - jacket route instead of the cheat ability - plated armor, that seems to be superior in any way. I can respect that, even though I wouldn't go for that kind of girl, if I would be in his stead. But I do respect that
His mom has been a pain for him since the beginning and might be in a hyperactive overdrive. He is her son and he can judge this the best. Both girls are onlookers and don't know the family business between mother and son. Maybe he has to stop her from time to time.
Sure, it is ugly to blow up in front of your friends, but his mom touched his stuff and ruined it. And it does appear she invades his life that all the time. I'm not talking about the inversion a parent should do into the lives of their children, I mean the sphere of private he should have as a high-schooler. You know, the "I'm not dealing with shady people and I'm not staying out too late without telling you. I also tell you where I go, even If I won't tell you what exactly I'm doing, but at least I really don't lie to you and you reeeeaaaally have no reason to worry."-kind of earned privacy. The MC give of a vibe of a non-endangered teen.
His mom is always disturbing it and he has all the right to snap. And both girls should be aware of it. And they are, as they told him of in a rather mild manner. Still no reason to shame him that much.