Poa has to be invoked by certification from a doctor. That's the way this particular document has been done for the circumstances.
OK, I see. My own POAs (registered in 2015) require certification of incapacity by a doctor, that is what I want, and I guess that is what your friend wanted. In your original post you wrote that you were in the process of setting up POAs, so I took that to mean that they were still being drafted or had not yet been registered.
If the POAs have already been registered with the OPG then you have to go with what they are, unless the donor wants to revoke them and start again, which they probably don't want to do. Apart from anything else that would be expensive.
If, however, the documents have not yet been sent in to the OPG and their fees have not been paid, the donor would be able to sign replacement POA documents that could be registered and activated without a doctor's certificate.
Do you already know which doctor you would be asking to provide the certificate? Does your friend already see a psychiatrist via the Community Mental Health Team or similar? If not, then you could ask a GP to sign, but I believe that not all GPs are willing to do this as they are not mental health specialists.
I guess you are wondering how to judge when incapacity has been reached? In your situation I would be asking myself:
"Who will I get to sign a certificate of incapacity and, if there is no obvious person, how do I access a suitable doctor?" The time may not yet be right to activate the POA but, since it could take a while to do so, it wouldn't do any harm to have the conversation with the doctor in advance.