I'm really hoping for some good luck for a change.
There is no way, whatever your experience, you can know for sure what it is yet. As NP said , they will have to wait for the scan.
I played with a mate that did his cartilage, it was almost like a gun shot, swelling and a lot of pain. That isn't necessarily as long term as ACL. There are many things it could be and we won't know until they have a scan.
I'm hoping for the best, sprain/strain and a couple of weeks. They are all painful, they could all give similar symptoms . Will be a real shame for the lad if it turns out to be ACL.