If the people you're with know about your diet and you're comfortable with them, then I'd suggest trying to stay 100%. I had visitors one time and "we" ate out a couple of times, but I just sat there with plenty of fizzy water and a coffee and it really didn't bother me anything like I thought it would!
If you don't think that's possible, and you absolutely can't miss the occassion, then I'd suggest having a soup - preferably chicken and/or vegetables, but avoid sweetcorn and noodles if you can. Perhaps tell people you've got a bit of an upset tummy (not exactly untrue)? If you need anything more, see if you can get some grilled meat of some sort and a bit of side salad - just a starter size portion should do you. I would absolutely avoid all the carbs you can - chips, rice noodles. As Lucy said, if you do have rice, only have boiled.
What would be great, if you get the chance, would be to pop into the restaurant beforehand (or try and turn up a bit early) and see if you can have a quiet word with the manager/chef - see what they can suggest...
Best of luck,
Marianne x