attaching to a spotter scope

A colleague at work has just announced he is taking early retirement and is a keen bird spotter
I know he has a spotter scope - don't know the make or model - and he has a crop Nikon (we all make mistakes). It would be great if I could get him an attachment for his Nikon onto his scope
Can anyone give me an idea of where I need to look and what I should expect to cost (based in UK) - are all scopes fitted with the same attachment or do I need to know the exact make/model he has
Any advice will be greatly appreciated

thanks

Dave

i did this with my 77mm Leica spotting scope I had to use an adapter made for the scope. Focus was difficult with this setup and long ago I got rid of the scope.
The bird forum www.birdforum.net is a good place to look for info on doing this.