Is it ever okay for a bf/gf to stay the night at their friend's house that is the opposite sex?

My bf has done it when she threw a St. Patrick’s Day party at his female battle buddy’s parent’s house and everyone was drunk including him so he spent the night along with his brother and a few other people that were too drunk to go home.

Yesterday my bf and his bro spent the night at her place again. Since they don’t have a car they took a train to meet this guy at this one place selling my bf a motorcycle b/c he was thinking of buying it. They didn’t get it and it was late so the busses weren’t running and his female friend picked them up but then got a flat tire and couldn’t take him home so they stayed at her place again.

I didn’t hear from him all day yesterday until this morning. He said he was busy yesterday cuz she took him to go buy his motorcycle. I don’t think he cheated…she just got engaged which does make me feel a bit better but I feel like a guy shouldn’t stay at a girl’s place cuz something could happen eventually. Am I right?

Edit* I’m afraid it starts off innocent and then as he spends more time with her he’ll fall in love. He’s already spent a lot of time with her (other girls and guys too) in the army and training and hasn’t changed. He does seem to treat them as one of the “boys” but they are kinda attractive. Me and him are long distance at the moment. been together 4 years 1 year long dist.