If she has not already seen a doctor or physical therapist, she may want to. I have found with clients with issues such as these that it is not the gluteous maximus that is always the culprit. For one it was a calf (soleus and achilles) issue. For another it was an adductor, gluteus medius issue.
You can also check her gait when walking. Some people, especially women, have a hip fall issue that can cause knee pain and stability issues over time. I’ve even heard of women having this issue due to quad VMO contraction issues. A physical therapist friend and I were just talking about these issues a few months ago and ways we can proactively help.
As some one suggested, a foam roller will help her to be able to identify where the issue may be stemming from, but an assessment with a physical therapist or orthopedist will help immensely!
Good luck! And take your time and encourage her to go slow. Most of those problems stem from long term issues and as such can take a long time to correct.