Private rental of holiday property on one-off basis

I have had a holiday property in El Campello for three years now. When we first bouight it we briefly toyed withe the idea of generating some extra cash by renting it out while we weren't there. But we decided that it needed to be a home to us and this included being able to leave personal stuff where we wanted it rather than locked away, and to be able to furnish it our way rather tha the way some potential holiday renters might prefer it.

Now a friend of ours who is attending an all expenses payed language course in Alicante has asked to be allowed to rent the house. He is a very trsutworthy person, so we would happily let him live there.

However, the leghality of the situation is a cause for some concern. He said he wants to pay us a fair market rent. I guess he would ask for reeimbursement from his employer and this would mean I would have to provide an IVA number and pay IVA. Also there would be income tax implications as well as maybe being flagged up to have the house inspected for rental suitability etc. I like to do everything as legally as possible but I'm feeling this might cause a lot of extra work and hassle for me to achieve what is ultimately just a  service to a friend.

On the other hand I could let him live there for free and tell him to forget the reimbursement.

Or is there some middle way, some level of rental income that would legally fly under the radar?