Hi Amanda - I would suggest your friend to seek legal advice either from a solicitor or Citizens Advice.
The form that he would complete will depend on what rights he has and how they can be protected. Guidance on how a third-party can protect their rights on the title register can be found in our Practice Guide 19. However, Land registration is complex, designed to protect legal and financial interests in property and there can be significant consequences for any error. It is for this reason your friend should consider seeking legal representation.