CGT - 2nd home following divorce

[GT3cs]

Any capital gains experts understand the situation if a house was wholly owned and lived in by partner A who then married partner B who also then moved in . Several years later they divorced but partner A moved out and bought a new home with a new partner , while allowing now ex partner B to remain living in the house ( rent free ) until there death . There does seem to be some allowance for this in the rules but in this case Partner A moved out 20+ years ago and now it’s time to sell . I imagine quite a rare generous scenario ….