An interest in a CGT asset is itself a CGT asset.
I am assuming the property was not inherited.
While not physically living there, it is hard to argue the main residence exemption unless there is a legal deeming that it is your main residence.
If they initially established it as their main residence then the absence rule s.118-45 *might* allow the absent person to nominate that property (their share) as their main residence for up to 6 years after vacating providing you don't claim any other property as your main residence.
If it was merely an IP from the start for the absent person, then no main residence for them.
It all depends on specific details to your situation, so don't use this as a substitute for appropriate advice.
Cheers,
Rob