Some people can be so oblivious of boundaries. Like, you give so much elsewhere they don't even consider it's rude to take your things. Have had many like that. If she doesn't bring him mits though, I would have him go out one day with no mits, put your on a back shelf and when she picks up, let her know you couldn't find the mitts you usually loan him today and his little fingers froze , he will need them the next day. I have brought little ones out in the winter in their tennis shoes with snowpants because parents didn't come prepared, ticks you off.....the poor kid suffers cuz the parents dont care enough. How do you explain to a little one, they have to stand in the packed down snow of the driveway because the parent wasn't responsible enough to dress them proper? not fair!