Emily Yoffe gives some good advice on something that usually does not come up for most parents.
Briefly, her son's girlfriend never wears a bra, and it's evident. And distracting:
"My 20-year-old son "Ted" has a 19-year-old girlfriend named "Dahlia." Dahlia is very well-endowed and rarely wears a bra. However, she does wear low-cut clothing and often looks like she's about to fall out. The dress she was wearing last night was so small on her that it she couldn't zip it up all the way and she was very close to a nip slip."
Soon they are going on a vacation, and taking Dahlia too. Emily's advice is kind; necessary for someone who is obviously oblivious to ordinary social conventions:
"You have time to have a friendly and helpful chat with Dahlia before you all go bouncing off on holiday. Take a supportive, not punitive approach:"
[Buy her a few bras.]
Going bra-less is acceptable in some settings; but not in others. Part of growing up is learning the difference.