Hi Selfsoothe,
The best route is normally via referral from a General Practitioner (GP) and or Psychiatrist. Could you go with your mum and see what her medical professionals have to say? Bi Polar is chemical imbalance, so here in the UK the NHS would not typically fund Therapy for that. Also Lithium is a powerful mood stabiliser so just changing dose needs carefully managing, so the big danger is normally patients coming off it or taking too much.

Here again you absolutely should speak with a Psychiatrist about that.
Child trauma can be co-morbid with Bi Polar, so it could be that Lithium and Therapy may be considered together. No one here, even the qualified, is insured to diagnose by proxy, and be wary of anyone that does. Listen you your Mums experts, and I wish you both the best.