He whipped my baby with a towel

My baby girl (1.5 years old) was playing with my bf. She had his hand towel and he ended up grabbing it and flicking her in the stomach. She started bawling and I checked it and there is for sure a mark/ welt so clearly he did it WAY too hard.

He is apologizing a bunch but I’m really mad. What should I do??? (No he is not her father)