Lol I thought the OP by ethical meant the fact that the daughter and husband are living rent free (on the taxpayer, so to speak, I know it's not correct because it's part of the army wages) while getting her mortgage paid by renters.
I don't have an opinion on that scenario but I have one on buying a property abroad to rent out: do not do it!
Renting out is hassle when you live near the rental, if the rental is abroad the stress would be immense!
Then factor in Brexit with new property rules, limited stay, etc.
Property maintenance people trying to fleece you, local bureaucracy.
Not worth it imo.
Your poor daughter, I feel for her if you convince her
