Honda are still making the GL1800, so spares will be plentiful - I can still get nearly all the parts for my 1979 GL1000! Can't imagine there'll be anything like the issues that current M3W owners are facing with the now obsolete S&S engine.
That said, they stopped producing the GL1800 in the US about 10 years ago. Does the ACE have a brand new drivetrain or is he using secondhand parts like a kitcar? Shouldn't be an issue - the Honda's are very well constructed, reliable and built to last. Owners won't face any of the issues that you get with the current V Twin.
(add - GL1800 mileages of over 200K are commonplace, with someone recording 425K)
