Stealth vs out is always a touchy topic. I think I can safely say that if most people could be extremely deep stealth with zero risk of being outed they would be. But life does not have absolutes, so the risk of being outed vs the risk of people prejudging you because you are trans is something your friend will have to weigh.
You can likely never hide being trans from your romantic partner... goes both ways. There are people who say they hid it from their partner for many years, really? I would only believe that if they transitioned as child. People also say they forget they are trans. Sorry I cannot believe that either, especially for pre-ops. You're taking hormones, you don't have a period and if you're pre-op you still have your little friend down there. Post ops still have post op care and things like that. What may happen is that it doesn't preoccupy your life rather than dealing with dysphoria, and that makes more sense. So with that in mind your friend may have to be prepared to come out to or be outed to a partner sooner or later. When, is up to them.
Best of luck to your friend and I think it is awesome that you care for her.