One big problem with the management of impotence in the UK is that generally speaking, it is not taken seriously and the majority of doctors are ill trained and do not have the expertise or time to deal with it in a professional way.
This is not the case in many European countries and especially in the U.S.A.
If medication by Viagra or it's derivatives doesn't work, then treatment usually comes to a halt.
The link by Buenchico 20.05 gives a comprehensive but basically academic approach to the problem, one designed to give a candidate 100% in an examination, but not a practical assessment of what is available in the UK.
Sexology has never taken off in the UK.
The professional management of impotence is only available in the UK in the private sector (no pun intended) and even so, in areas where Urologists have a particular interest in impotence.
Patients must not be drawn into unrealistic expectations in the management of impotence.
My advice is to your "friend" is to make inquiries about clinics that deal with such matters, outside of the NHS.