You are focusing on the wrong thing. Rather than focusing on your ex, focus on DD not being provided with the support she needs and is entitled to.
If DD can’t attend school because of her MH, the absences should be authorised. Email the school requesting the absences are authorised. State the regulations (Education (Pupil Registration) (England) Regulations 2006) make clear where a pupil is absent because they are unable to attend due to their medical needs the absence must be regarded as authorised. Also include DfE’s attendance guidance states absences should be recorded as authorised where pupils cannot attend due to illness (both physical and mental health related).
Go on to say that as DD is not receiving an alternative education under s.19 of the Education Act 1996 and an EHCNA has not been requested, the school and LA could have done and still can do more to support DD with her additional needs. Finish by giving them a reasonable deadline to reply. I would copy in the headteacher and the Director of children’s services at the LA too.
In addition to this, the LA should be ensuring DD receives suitable alternative provision so that she receives a suitable full-time education. This should have begun once it became clear 15 days would be missed. A diagnosis is not required. Email the Director of Children’s Services requesting provision. If they refuse, delay or ignore you, email again reminding them of their duty under s19 of the Education Act 1996 and if they fail the comply with their duties you will be forced to look at judicial review proceedings. Then, if that fails, contact SOSSEN for help with a pre-action letter.
Alongside this, request an EHCNA. On their website, IPSEA has a model letter you can use. Again, this is based on needs, not diagnosis.
If you post on the SEN or SN boards, you will receive advice about EHCPs and alternative provision.