Renting / Letting options for an old (non compliant) house in England

Hi, I have a relative who lives in a very old house that does not meet current regulations for renting out, yet he can’t afford to bring it up to spec.

With that said, someone has reached out to him to say that they would like to rent it irrespective of the condition, as long as there is a reduction in the rent ?

My feeling is that is not legally possible to do, however could something be done where the rent deductions was position as a way for the renter to bring the house up to standard? (If the work required costs £5k and the rental for the year is £15K, only £10k rent is payable in the first year)

Ultimately I’m looking for options for a non-compliant house which do not involve any upfront payments or loans as it’s not something they have or can do ?? Also the local council do not provide any options either - so sadly I’m all he’s got for advice ..