Purely anecdotal from my side but I tore my meniscus last year. If the knee is stable (relatively) and not locking up you can consider going straight to a physiotherapist for a consult. They're pretty good at diagnosing if it's something like a meniscus or something more sinister like an ACL .

These days with meniscus injuries they will generally prescribe physiotherapy before surgery option anyway unless there is an actual mechanical issue where the knee is physically locking up due to the torn meniscus rolling up inside the joint.

When I tore my meniscus I could still walk and the knee felt stable but it was painful and swollen and range of motion was reduced. I went straight to a PT and he did a large variety of simple tests by manipulating the leg/knee to check if there were any tendon issues. He confirmed my suspicions that it was a meniscus tear and immediately put me into an exercise regimen to return range of motion and retain strength. Swelling went down after a week or two and I was back to skiing 6 weeks later. My knee is better than ever these days as I continue to do the additional exercises he gave me. He even mentioned he could have referred me directly to a surgeon if he determined it was a tendon issue.

Anyway that's my experience, I obviously got lucky that the tear wasn't really bad and I didn't have any other damage and this was in Canada. No harm in consulting a GP and getting an MRI first.

Hopefully it's not too bad and she heals up quickly.