As someone who has had treatment for anxiety disorder plus has had a child go through CAMHS can I suggest/point out the following:_
If an anxiety/panic disorder or depression is diagnosed, medication to manage the symptoms is often prescribed whilst an appropriate course of psychotherapy is undertaken. Its very difficult to treat the problem whilst the person is experiencing the symptoms (you wouldn't have a broken leg reset without hefty pain relief would you?) but most decent GP's would not look to this being a long term solution, especially for someone so young.
Traditional 'sit down and tell me about your childhood' type counselling doesn't work for everyone. There are also various types of counsellors who employ different techniques so get DH to find out what she didn't like about the first and see if an alternative approach could be used.
CAMHS often has a very long waitlist and if she had only just been referred before lockdown she may well be waiting a while yet to be seen. Plus, tbh, CAMHS can sometimes have a very strict criteria so they may say refer to GP for medication and local counselling service for psychological support.
It may be worthwhile looking into local young adults services that can offer support / advice. My eldest had some sessions with a peer supporter - someone who was only a few years older than him - he was more willing to do this than sit with a 'stuffy old counsellor who didn't understand him'
The drinking and smoking weed whilst taking medication is a concern - alcohol and weed are depressives so would counteract the benefits of most medications.
The only other thing is - you obviously care for this girl which is wonderful but unfortunately there is little you can do yourself. When my DSS was having some difficulties I couldn't talk to the school or anything about it, despite the fact my DP has full custody and his ex has virtually nothing to do with her son. Best you can do is give DP info and let DSD know you're there for her.
Good luck and I hope she gets some appropriate support.