Housesitting for a friend, and I think I’ve killed their plant

Please help! I think I've possibly over-loved this guy in my friend's absence. He looked great just a week ago! My theory is overwatering/root rot…some of the roots seem exposed and it's in a mason jar. Maybe too cold by the window (we're in Pennsylvania, it's dipped below freezing at night)? My friend is super sad about this already, and I'm hoping to try and reverse the damage if possible.