To answer the first question, if you receive money (rent) then you will have to declare it. It is taxable income.
You would be wise to setup a tenancy agreement, just in case. If your Don and his partner/wife were to split up the wife would not be acquiring any proprietary interest. You need to set up an assured shorthold tenancy agreement.
I hope this helps.
Do you have any further questions or queries?
Happy to help further.
Did this help. Happy to help further.
If you do not require further help can I please ask you to accept /positive rate my assistance so that I receive credit. Thanks.