It's a question of balance, I think. The performer has to appreciate that the folk club is probably a big thing in the life of the organizers - something they're proud of, hopefully - and a guest showing an interest in the activities of the club and its residents will be appreciated.
The club organizers also have to appreciate that while they're club is an enjoyable night out once a week, the professional performer is doing his stuff may 3 or 4 times a week. If you book the guest for 2 half hour spots, don't expect him/her/they to be "on" for the complete evening and the almost obligatory after gig party as well. It may not be quite like "a day at the office for them" but at least accept the fact that if they put in a good performance during their own set, that's all you can reasonably ask for. Anything else is a bonus.