This is what's bothering me. Edit= and nothing I'm worried about.
That's pretty natural then. Unless she has been friends with the guy for a long ass time, it's weird to go on a road trip with a straight male friend.
It's all circumstantial though - usually, you will know who your SO's friends are and have a good feel/trust for them. You should also trust your SO. If you know and trust one of her straight male friends then I don't see the problem with her going on a road trip with that friend.
If you don't know them at all and you've been dating a long time? That's a problem.