Keep yourself in hand and try to stay calm. There is nothing worse than to snap up the former with the charges or, conversely, with excessive arms.
Don't try to compare myself with a cute companion ex-boyfriend, if he met you one. You can't know all the details of their relationship. Maybe she's his lawyer, realtor or boss with a bad temper. Or she's annoying and overprotective mother, who moves in with him tomorrow to live. In the end, you know how wonderful you were, and I would think your ex – who are now worried about.
Do not worry about politeness. If you understand that you will not be able to communicate with your ex quietly, nod and walk past, even if it stops. In the extreme case, explain to him that in a hurry. If you want to talk, pick the most neutral topics: mutual friends, Hobbies, etc.
Don't be too curious. Think about it: do you really want to know who he is now found and lead their new friend to "your" restaurant? Surely without this information you will be safer to sleep in the next few nights. Yes, and keep from having to start to look for the information about it in social networks. Survive at least three days, and then this desire will be gone for sure.
Not wait if the conversation ceases to like you. Begin to feel sad, jealous or angry, say your goodbyes, cute smile and leave. He talks about his successes, or failures, and you don't want to hear it – just nod and go about their business. The former begins to mock you, to mock and be rude? Just silently turn around and walk away, don't waste your time on him.
Use a meeting with the former as an excuse to think about his own life. Analyze what you liked and didn't like in your relationship and how it can affect your future men. Remember, you might have some classes or people who really liked you before, but you left them because of the breakup with your ex. Now you can go back to it, it will surely bring you pleasure.