1. There are a lot of dodgy outfits doing sale & rent back. Many only give a 12 month tenancy & there is absolutely no certainty that you won't be evicted when that tenancy ends. Also, she may well find she can't get Housing Benefit to help with the rent, if that is what she wants to do. General advice - be very very careful & get detailed advice from CAB or a solicitor before going ahead.
2. If you can use the above to put her off the idea, then you could propose putting the house on the market before it is repossessed. That way you will get the best price for it. She will have to find somewhere else to live, but that's going to happen if you don't agree the sale & rent back because it will be repossessed.
3. I don't know the answer to your question, & doubt whether anyone can tell you. But it could well be that the value the sale & rent outfit uses & offers 75% of is not a proper full market value anyway.