Have you checked to see when they were last active on Ancestry?
If recently, it might suggest they have more than a passing interest in their tree (ie haven't just put together a tree, taken DNA test, then never to be seen again), so might be more open to discussing how your shared cM has potentially come about, and crucially, more understanding of/prepared for the fact that once you go down the DNA route, you are opening up the potential of discovering something that might never have been discovered with manual research alone.