Gez,
Here in America we have restaurants that are open all night called "diners" - there's no extra 'n' there. And he wasn't out with another woman by himself, he was with a friend (a guy) and another woman. It's not unusual at all for people of all ages, but especially younger people (especially those in school/college/university) to get hungry at 2am, even if they ate at the normal time. I can't tell you the number of times I found myself starving at 3am when I was out with friends - I spent many a late night at the diner down the street from my apartment with my friends (all guys) even if my boyfriend at the time was staying home or wasn't there. I never did anything untrustworthy.
This guy has a right to live his own life and to be trusted by his girlfriend - as long as he stays trustworthy. It's not surprising or strange to be at that diner at 3am - there were a lot of times where other people suddenly decided to go to the diner and I tagged along. A diner isn't really romantic or a place for a date and his story is more than truthful.
Trust me, if you were over here, we'd be sure to drag you out to a diner at 3am too!
some of it's magic
some of it's tragic
but i had a good life all the way......
~jimmy buffett