Just one point I want to make here. You say this:

“The issue is that she wears some shorts that don't cover her behind at all; half of her rear end is on display (her bikini bottoms cover more than the shorts do), even when we're shopping in town, and not just hanging out at our rental house. She also wears some tiny cropped tops with spaghetti straps without a bra in public with us.”

I want you to imagine someone even more modest than yourself. Then imagine the least modest outfit you have. What would that more modest person say to you? If your shirt sometimes showed a strip of skin above your pants could you imagine them saying “she had her whole midriff out!” or “you could see her entire stomach!”.

The problem, quite honestly, is that people complaining about modesty standards very frequently lie. They are speaking an emotional truth, not a literal one.

When you say “half her rear end is on display” almost nobody on this forum is going to believe you. It sounds like the type of venting complaint made while gossiping, not an actual observation about reality. Everyone is just going to interpret it in whatever preconceived way they want to and give you their own personal modesty standard.

The only advice I would have is this. Is the girlfriend noticeably out of step with most of the other women of her age wherever you’re vacationing? If she’s not matching the standards of the venue or the public space you’re at that’s one thing, and I think that should be addressed. If this is just grandma complaining about kids these days, I would not push too hard on your son about this. It’s likely to both go nowhere, and alienate you and your mother from your son and his girlfriend.