Bf cheating??

So last night my bf was drinking and was gone with his brother. And i found out that he flirted with his ex which is his sister in law now. Note: they dated like years ago! Anyways, he said he doesnt even remember last night. He didnt know what he was doing. He drank like 3 40s. Idk what to do? He didn't know what he did cause he was drunk. Idk if i should just forget it or break up?? He said he doesnt see her like that. Their relationship from years ago was nothing. She used him for food pretty much. Should i end it or stay? He said hes going to stop drinking too. And he wishes to take back what he did and is sorry..