"in the UK and EU firing someone for anything other than gross negligence is very, very hard"

It certainly isn't difficult to fire people in the UK providing they haven't been working for you for too long (it's either six months or a year). Other countries in the EU can be a nightmare though.

Typically it's laid off or terminated without cause, in which case the person gets unemployment, the business takes a hit on their unemployment insurance but it's typically no hassle as long as it wasn't for a reason that's explicitly against the law, like discrimination.

"Terminated with cause" avoids the hit in unemployment insurance to the business, the person doesn't get to collect, but they can sue you for wrongful term if it really should have been the first category -- here's where you need to document gross negligence.

Do they not have contract to hire in the EU?