A couple of possibilities:
As you can imagine wear and tear catches up with you.
So true. Part of getting stronger almost invariably involves the loosening and weakening of joints, whether you're running, squatting, whatever. At 30, I'm stronger than I've ever been, but the years of rugby, lacrosse, triathlons, and other nonsense have taken their toll. The best way I've found to counter loose joints is to (1) strengthen the muscles around the joints and (2) develop an understanding that the lion's share of the work should be done by your muscles, not your ligaments, when you're doing anything physical.
Here's a quick, basic progression to activate and strengthen stabilizer muscles that will mitigate the likelihood of future injury & further loosening of hips & knees.
Nothing too fancy - all you need is a foam roller or a hard rubber ball and an exercise band. I gather some of you are into that super bare bones, old school fitness stuff where you go off to a forest, kill a mountain lion with your bare hands, nail its carcass to a tree and use its body as a pull-up bar, so hopefully you'll forgive me for suggesting the use of such haute couture equipment.
Pre-progression
10 min myofascial release: loosen up muscle tissue that may be restricting your hip movement. Apply a foam roller or lacrosse ball, esp to your glutes & the area right under the lliac crest (top of the hip bones). A lot of weak psoas & hip flexor issues have to do with tight glutes. Tons of videos/articles on proper foam rolling.
Progression - 2 sets
1)
lateral banded walks, 20 meter each direction: These primarily activate your glute medius, which by default is unused by most people thanks to our cowardly-ass sedentary lifestyles, as canuckroyal mentioned. Super good for hip strength.
2)
10 banded good mornings: Gets those hamstrings activated.
3)
Walking lunges, 10/side: pretty self-explanatory.
4)
10 burpees - to apply the glute & hamstring activation to a compound movement. Smooth & with good form. Think about activating the glutes and hamstrings on the way down and up. Also, she'd get it (watch the video).
Hope that helps!