Hi,
What purpose does your friend have in mind in sending a letter at this stage?
On the face of it, sending such a letter would have no material affect - particularly with the forthcoming transfer of IB claimants to ESA under which there are no exemptions on mental health grounds.
I think most people would take the view that in relation to IB/ESA it would be better to wait until your friend is actually selected for review, which may not be for some years.
In relation to DLA the answer would probably be different.
Good luck,
Steve
Nothing on this board constitutes legal advice - always consult a professional about specific problems