My brother had this when he was younger(he's now 25),he loved playing football but his knee kept 'giving way' and he was diagnosed with it after a few tests.
His was pretty bad he'd play football,his knee would give way then he'd end up in a plaster cast from his ankle to hip for 2 months ,till it was better ,before then playing football again and it going again...so on so on
It bothered him quite a bit as he was very sporty,and it got to the stage where he could just be walking and his knee would go.
One time we had all gone on a family holiday abroad and he walked down a step by the pool and it went,he ended up hobbling about for the rest of the week.
The only thing that really helped was hot and cold treatments,ie a heat pack for 10 mins then a cold pack for 10 mins .
In the end he had surgery to correct it(when he was 20),they first tried keyhole surgery but he then had to have open surgery on it,as the first op wasn't successful.
He had to have lots of physio and can now run on a treadmill,play light football (with my dc) but would never be able to play competitive football ,hard sports again,which is sad as he was a great footballer and loved it so much.
Although it's not cheap I would recommend seeing a sports physio,my brother visited one (friend of a friend)regularly when he was at his worst,and he did say it helped alot.
I hope your Dd feels better soon ,it's not nice for anyone to get ,but when its a sport-minded youngster then its really sad.