Although I agree with SilverAvion about perhaps trying the trail first, in Phillip's defense I work in medicine and our protocol is to recommend exercise and other cardiovascular strengthening activities for hip dysplasia, especially walking/hiking, cycling ... well, and swimming. Furthermore, even though impact sports are not recommended, many patients still do it and some even have run marathons with this condition. I would just use common sense. She should know her body best than any physician would. She may already be taking NSAIDs I assume.
I also wanted to confirm that your friend is human and not a dog :-) ... as this condition is common in dogs, arguably more so than humans.