Helping house

My client and his wife are moving from their present house which has always been used as their main residence. They have purchased another property which will then be their new main residence.

My client wishes to let his father occupy his old house for the foreseeable future and I understand that the father may also take a lodger into the property. What is the best way to mitigate capital gains tax on a future sale of the old property? My understanding is that the client will not charge his father rent and this makes me wonder whether the residential lettings relief will be due.

I did wonder about whether the property could be held in trust for the father so that relief might be due but obviously the client would like any proceeds from a future sale of the house to be legally his rather than his...