How would Mycroft react when he finds out that his SO had to sell their body to help with the rent when they lived with their parent/s, when they were young?
Concern about her mental and physical well-being would be the first one.
Obviously this isn’t something that is healthy for a minor (because if they were older and lived in a more urban area the chances of them getting a regular job would be higher even if the pay for being a sex worker is infinitely higher.)
If Mycroft can gauge her mood on the subject he might ask her story and what he can do to help her.
Therapy is definitely going to be an open option for her should she choose to seek it. She may not want it at this time and that’s fine.
Mycroft will probably want a list of things that are good and not so good to do with her as not to bring up bad past experiences.
He’s probably going to want to come after her parents for not being able to do more so she wouldn’t feel obligated to but then again that’s up to whether or not she gives Mycroft the go ahead.
Its not going to change the way he feels about her because like addressed in other posts you have to do a LOT of bad shit in order to get Mycroft to stop loving you and Mycroft is always about helping when he can.