After my husband went to bed, my friend told me that he had said a few inappropriate things the night prior, including "confiding" in her about how he's not happy with our sex life.

So did you actually sit your husband down and talk about this?

Since then, I know my husband has messaged her directly a few times- nothing obviously inappropriate though.

How do you know? Do you regularly check his phone or do you just think your friend would tell you?

As I walked out, I saw them walking up the hill holding hands.

I think you simply didn't find the inappropriate messages.

so this isn't a "is he cheating on me with her" type post

Haha, what? C'mon, why should they be holding hands? There's more going on.