ds girlfriend crying loudly last night how do i handle it

[frostbite]

my ds 17 nearly 18 has a lovely steady gf also 17 and they stay over in each others houses two or three nights a week,have been very close 6 mos.Last night at midnight heard her crying really loudly in his room [has never happened before and she is very cheerful and happy usually],then some conversation or arguing with her still sobbing and my ds saying sorry a few times.
This is very unusual and obviously i want to know whats happened,or at least what sort of thing happened.Want to talk to him tonight about treating her well[he left the table without her last night and seemeda bit off to me],but want perspective as i think i might say too much/get cross as i feel worried and upset about it.
She is very sweet and sensitive and i fear he has hurt her feelings/been off as his social skills altho much better than they were,can be lacking sometimes ~ he is very sweet but can be a bit thoughtless and i could imagine,could drop a gf like a hot brick if he went off her.
It seems odd as they were happily with us all evening.
I don't usually get involved but feel a bit concerned.Am definitely going to talk about treating her well while here,and how if they do finish,to do it nicely as she is very in love i think and v sensitive, she has also been thru quite a lot.

[NewApprehensiveBeginning]

That's great that it wasn't anything too serious.