CGT on property for with ex-wife still joint owner

My parents (retired, age 70ish) divorced in 1998. My dad remained in the family home and my mum left, remarried and jointly bought a house with her new husband around 1999. My dad remained in the house until now, his only property, but soon needs to sell (to settle an interest only mortgage he took out around 2004). My mum never came off the deeds to the property so is technically still a joint owner, although has not lived their or made mortgage payments at any time since she left in 1996. If my mum “comes off the deeds” or signs over the ownership to my dad, does he become liable for CGT? If she doesn’t come off the deeds, as she jointly owns another property, I’d assume that when its sold she would be entitled to half the sale price, and liable for CGT on her share? Grateful for any guidance, clearly it’s unusual.

Hi,
If your mum is a beneficial owner of the property and will recieve a share of the disposal proceeds, then she may have a capital gains liability from the disposal.  
If your father will retain all of the disposal proceeds as the beneifical owner, then he will have to work out if his Private Residence Relief covers all of the gain. You can see guidance here: HS283 Private Residence Relief (2024)
There is a calculator for capital gains, which links into reporting and paying any Capital Gains Tax due, here:
Tax when you sell property
Thank you.