What is the EPC rating for the flat? If it's impractical or impossible for you to improve it, then it is what it is, and she needs to consider moving. Good luck on finding better, because many landlords are getting out of the rental market precisely because getting their EPC ratings up to rentable levels is impossible, impractical, or just not financially viable. I gutted a 1935 semi and invested hugely in insulation, eg. 300mm of insulation in the loft, double glazed windows throughout, etc. ... and the best EPC rating I could manage was C. This'll enable me to keep renting it out for the near future, but if the powers that be ever insist on better than C, I'll simply bail on it and sell it to an owner occupier.