Poor little lad
Could you ask them if they could pay to get him neutered first before rehoming him? It might make it easier for them
Is there any local rescues near you that might be able to help?
I really feel for you Nat - it strikes me that as soon as some folk know you love buns, that you'll take on any unwanted ones
Good luck - I hope your aunt's friend can get sorted with a new forever home for their poor bun