It's their right to do what they want as long as nobody get in trouble BUT the one who's gonna get in trouble is that girl because Thailand has a very very very good and beautiful culture, a lot of rapists and teenage moms. Just only one pic could effect her a lot.
'Don't tell us how to dress. Tell your son to not rape.' Is a good quote but there's a few Thai people who give a damn. I can't say it's okay to dress like that but I can't say it's not. The question is 'how it's wrong to wear bikini?' I think the answer can solve your problem. Just find it by yourself because it's just an opinion. No right no wrong.