This is a situation I was in a few days ago, and wondering if I could have said anything without being rude.
Setting the scene:
I was at a busy restaurant at lunchtime with a friend. It's set up fairly informally, with long tables and benches, with room for about 20-30 people at each table (Wagamama's for the UK e-hellions). When it's not busy they spread people out, but obviously when it's busy you're put wherever there are enough spaces for your group. My friend and I were sitting about in the middle of the table facing each other, with a group of four on one side (and another group on the other side, but they're not relevant to this story!).
One of the group-of-4 started talking about how he'd been sick recently. Flu-sick. With descriptions of symptoms. Graphic descriptions
I was sitting right next to this person, and there was absolutely no way that I would not have heard. To be fair to him, he wasn't talking loudly - if it was a normal restaurant with separate tables I'd have been very surprised if anyone had been able to overhear. But it's not like they didn't know we were there.
As it happened, we had finished eating and were in the middle of deciding about dessert so it didn't actually ruin my meal (although we did decide to just pay the bill and leave).
Would I have been OK to ask him to change the subject? I'm generally squeamish so could just be being oversensitive to the subject matter, though my friend was a bit disgusted as well. If it matters, usually when I eat there people tend to make eye contact with their table mates, maybe a friendly head-nod. There was nothing with this group though, so I'd have been coming out of nowhere.
This has turned into a very long post, about quite a simple question! The line between overhearing / eavesdropping is narrow though, and I really don't want to cross it.