How do I get my friend to stop crashing out?

Sometimes when I beat my friend in this game, he rage swaps to hardmode sans (yes, I play with my friends mostly in private servers). I can get him down to phase 2, but past that, I dont beat the 2nd. Due to how I play, I get hit alot, so no hitting will definitely be difficult. Anyway, anyone have any suggestions on how to not make my friend crashout when he loses