This exact scenario was posted in the summer with some identical idiotic responses about it being ok because women look at men and it was probably to show off in a WhatsApp conversation as though these make the surreptitious photographing of someone's body acceptable.
Like all people with a functioning brain, I'm careful when taking photos on the beach/at the pool on holiday because I respect the privacy of others and I wouldn't want to make anyone feel uncomfortable. If people are sunbathing topless, I change the angle and don't take a picture of them! It is not ok.
I would hate to feel like I can't relax on the beach in case some inadequate creep is sneaking photos. It's not justifiable behaviour; it is pathetic. If men want to see breasts, they are hardly difficult to find images of online. So the men who do this specifically enjoy violating women's boundaries - the lack of consent is something they are especially seeking out. It's revolting.
And if I had a female friend who photographed an unsuspecting person in a state of undress and sent it to me whether it was to make jokey comments, be salacious or make fun then I would give them extremely short shrift and tell them to grow up.
I would imagine most hotels would have guidelines about photography for exactly that reason. OP, point out to your partner that if the woman had realised she was in his photos, even in the unlikely event of it being accidental, he could be asked to leave the hotel. There was a model in LA who photographed a woman in the shower at a gym and captioned it with a snide comment about her body and shared it to Snapchat - she's banned from every branch of that gym worldwide for life. I would really hope hotels would do the same for any guests who were snapping sunbathers without their consent or knowledge, especially if they were sharing them on social media for their pathetic 'lads banter'.