Interesting question and scenario, thanks for asking!
Stress by itself does not make you better looking, in fact it typically has the opposite effect. What is stronger than the stress, though, is belief.
A good way of internalizing all of this is to say that mind is stronger than body, and will is stronger than the mind. Beliefs are things of mind that create will, so long as she can maintain the belief, the belief will create will that will override the rest of the mind and by extension the body.
Stress is an example of mind affecting body. Stress assaults your mind’s ability to keep track of and maintain your body. So people who are chronically stressed start to over time look worn out.
What a lot of people don’t realize about health is that it’s 90% mentally-driven, because the mind is more powerful than the body, very few ailments don’t respond to mindset.
Your friend is using beliefs and will in a concerted effort to keep up appearances while she’s being racked by anxiety. Anybody can use their will for anything, they just have to believe it’s possible and then just keep applying will for as long as it takes.