Getting there age wise and I have to disagree, lol. Peeps are not being immature, it is rage bait and people are biting. Small but important difference.
MC might be in his right to allow the leech (would say the same if she was male, btw) to feel better in his own home but she factually is a terrible human being. Even more so if it really is not his kid. But even even more so if she is and the mother kept her from him.
And, again, even more so if she really is not doing any chores or anything at all for him offscreen (nothing on paper for now). She is, for the reader at this point, the worst kind of freeloader. Prime rage bait.