Wow. I mean, I am not religious either. But to say it's disrespectful for a group of Christians to quietly pray at a table before dinner? My dad and stepmother do it too. Why is it such a big deal? Who cares if they get looks? They are respecting your right NOT to pray, and you should respect their right to do so.
Were they asking and pushing you to join in, or the waitress, or being unruly loud about it - then it's a bit disrespectful. But can't you just give them a minute to do a quiet prayer when it obviously is important to them? It seems that, unless I'm missing something, they WERE keeping it to themselves.
I see you've already decided not to dine with them again - so I won't go into the "if you don't like it don't go" bit I would have otherwise. But I have seen many other things I would consider offensive - the situation you were describing certainly does not fit the category!