Thats it ..and If what you said at the end is it then fine ...To summarise.
Your relatives own a 2nd property. It's currently vacant, not rented and they don't need the rental income. They don't have any intentions of selling the property.
They've been approached by a couple who would like to rent the house from them.
If they hadn't been approached, they have no interest in renting the property.
This doesn't sound like the usual case where their biggest concern is not getting rent paid on time? So what is the concern you're trying to protect them from?
Just make sure everything is above board. Registered with RTB, utilities in tenants names, correct insurance in place, and tax returns filed correctly. They can ask for 2 months rent as deposit.
1. The tenent stops paying the rent ..they dont need it but it may cause stress.
2. They decide or need to sell in 2yrs ..and having followed the process the tenent won't move