I just got back from dog sitting. It's a bit different, I've been these people's only dog sitter for three years running (a nervous dog of theirs absolutely loves me, so it works out) and I was told to help myself to absolutley anything. Last time I was there, they texted me beforehand telling me they'd just bought a new amaretto and I should try it out and let them know what I thought.
I found out yesterday that the gin I had used to make a gin and tonic for myself this weekend was a very very expensive one. Honestly, I had forgotten that mom used a sliced orange in her gin and tonic, and I ended up tossing it down the drain, as it was too dry for me without the orange, but I felt horrible. It wasn't much, but it was enough that I felt bad for wasting it.
OP, if I had been house sitting and not know the value of the wine, I'd want to know. Don't couch it like you want them to replace it if you're not dead set on that, but just give them a heads up for next time.
"Hey, friend, thanks for watching our cats. Just so you know, we don't mind you drinking wine, but next time if you see one that's wrapped, please leave it alone. We were saving that one for a special occasion, and it was a splurge to get it in the first place."
That wording makes it clear that this isn't a friendship-ending thing, but something that shouldn't happen again.