If one is invited to a friend's or colleague's home for a meal, and they stop to pray before eating, it is generally accepted that as a guest you show a quiet respect to their action, even if not taking part (whether if it is of your own beliefs or unbelief). But how does this work in a "neutral" setting such as a restaurant or public picnic? Should you show the same restraint/silence as in the "guest" situation, or is it okay to begin to eat your own meal, or even carry on a conversation with "third parties" that might be with you?
Even if you are in a public setting, you are still participating in this gathering as a member. I think that showing respect of other people's beliefs will allow you to be seen as friendly to them. The cost you is that you pause for a short period while this religious act takes place.
It probably will not be viewed as disrespectful to them. But for the small price of remaining still, you convey the message that you are aware of their needs and you are accommodating it.
Two cases :
If your friends/colleagues/etc are the ones praying, I would remain silent until they finish, as in a guest setting (very few people do extended prayers in a public context anyway, and I tell you that as a believer).
If you don't know them, do what you want. Once again, even as a believer, I wouldn't care if all the other tables were praying. I just wouldn't bother them on purpose, but that's it.