As a minister's spouse, many of us hear more than our share of criticism, perhaps not about us, but about the church in general and sometimes about other pastors. This can be especially difficult if your "friend" goes to another church and confides their criticism to you about their pastor in their church. I've even had people say to me something like, "I can't wait for our pastor to leave, so your husband can come to our church." And I've had people say to me, "Since you know So-and-so personally, maybe you can pass this along to them."
Looking at these situations objectively, the person is trying to triangulate you into their situation. And you can be sure that if they confide something like this to you, they'll also talk about you behind your back.
The best thing to do is simply listen. You can empathize with their feelings without agreeing or disagreeing with them. I promise you, you don't want to get involve, no matter how tempted you might be. Short of some kind of abuse, your best course of action is to tell the person that if she has a problem with the pastor, talk directly to him or her. What the person is doing, perhaps unwittingly, is spreading gossip. And we all know what can happen and how gossip can destroy people's lives, reputations, and ministries.
I've even had folks tell me what they've said to the DS about their pastor--and not in a good way.
Believe it or not, people may see you as part of the pastoral team or at least, a kind of influencer. After all, you do have direct access to the congregational leader--the pastor.
As the Bible advises: Be as wise as a serpent and a gentle as a dove.