Fan clutch?
 

Well my dad has a 00 S70 GLT and the AC will work for the first few mins after turning it on but then clicks off and blows warm air...

Could it be the fan clutch and if so how hard is that to replace?

any help would be great, thanks!

You should be able to reshim the compressor clutch.
This is how to do it when you get the compressor loose.
You might be able to get it loose and do it from under the car through the subframe and body hole next to the dryer.
https://volvoforums.com/forum/volvo-850-16/c-compressor-clutch-reshimming-6665/