Option 1 increase the rent by 10-20% not enough to make the tenants move but will increase cash flow
Option 2 Since she took the loan some time back, Get a friendly valuer to revalue the property and seek equity release from family.
Option 3 If they refuse visit HFCK with the new valuation and they will buy it from them and offset the difference.
I may be wrong..but then I could be right