If I assume you are a staff employee, and do not have management responsibility... here's what you can say in return to your boss:

"Wow, that's a lot of serious information. I can understand why you are so focused on these issues. One of the things I like about our company is how seriously they take confidentiality and the protection of our personal information. I appreciate that you consider my input valuable, but I really feel uncomfortable knowing this information about my peer and I really feel uncomfortable making any opinions or comments about their situation. I hope you understand."

I think commenting like this will help you feel better as well as subtly let your boss know that this kind of information shouldn't have been shared. Sometimes bosses are human too and they just need a peer to bounce things of as well. Sometimes they forget that non-management personnel should not be used in this manner.

Hope this helps!