British citizenship and language test

[yellow beard]

My mother is 58 and has an ILR (indefinite leave to Remain) in UK. Due to her physical condition she is in receipt of (1) PIP (Personal Independent Payment), with a mobility element being paid, (2) a disability element on her Tax Credit payment and (3) Employment and Support Allowance, in which she is in 'support group' which excludes her from requirement to actively seek employment.

I understand that language test requirement can be waived in some cases, namely:

you are under 18 or over 65
you have a significant mental condition
you have a significant long-term physical condition.

We believe that my mother may fall into the third category. I attempted to find how this exemption can be verified, i.e. should it be declared prior to the application to be verified and signed off? Could gurus and members who dealt with this issue signpost please?