Asking Eric: Our friend's adult child sits in the middle of every gathering and never talks

Dear Eric: We are an empty-nester couple in our 50s. We entertain often, and in one group is a couple who have an adult child, W, who lives with them. This couple frequently asks if W is invited too. We always say yes. The problem is that W tends to put a significant damper on gatherings. They never engage in conversation, literally zero other than to answer direct questions.