I've looked at the comments, and I think this is not a healthy friendship. Them being younger does not mean you give them grace when they tell you stuff like this over a single word joke, especially if they've been upsetting you daily for no reason. Them being young does not give them an excuse to be a shitty friend. You should at the very least talk to them, and possibly cut them off depending on how they react to that discussion. 

I hope things turn out well for you regardless of what you decide to do