Is this cheating, what should I do?

My brother and his gf have been together for 5 years, until a few months ago they decided to have a "break" bcus of too many arguments... this break wans't a break up, just a time and space away but still together. So they never really broke up but a week ago she came over to our house and went out alone to see some friends... when she came back home, a guy dropped her off at 3am, so my brother thinks she lied about who she went out with. After that they had a bit of an argument and she said her feelings for him were not the same anymore and she is starting to see him as a friend... she went back to her town and my brother, I and some friends went to a party. On the party my brother made out with one of our friends and they probably had sex on the car as he dropped all of us first and then went with her alone... what do u think I should do? i feel bad for his "gf" bcus altho they are not well, they are not totally over, tomorrow she migt come back to him and will never know what he did 😩😔