The goal is to be loved and appreciated for who (you) are.
Generally speaking if someone has to change their (core being) to make a relationship work they may be with the "wrong person".
Clearly if your friend has a boyfriend she already has something that he was attracted to. She shouldn't try to "become what (he) wants".
It's impossible for her to be happy unless she can be herself.
No one should approach a relationship like their mate is a "project" who they hope will change into someone they are not.
You want someone who (already is) the type of person you want.
Having said that if (your friend) wants to change for (her) sake she might want to take some drama or speech classes/Toast Mastes, join some hobby groups on Meetup.com and allow herself to gradually pick her spots to open up.
One of the issues with being an introvert is they either worry too much about what others will think of them or they simply have no interest in others. In order to avoid the pain of rejection they simply choose not to engage. They subscribe to the saying:
"It's better to be silent and thought a fool than to open your mouth and remove all doubt." They would rather be "chosen" than to pursue.
If her boyfriend was "the one" you'd think she'd have no problem letting down her guard with him. Maybe (she) doesn't yet trust him enough. It's easy to relax and open up around people (you) trust!