Should he take his old girlfriend to dinner?

My boyfriend and I have been dating for 10 months. He's 51 and I'm 47. Yesterday morning (Friday) he called to tell me his old girlfriend was coming to town tomorrow (they only broke up because she moved across the country) and wants to have dinner with him Sat night. He says they are just friends now and have stayed in touch since she left by email. She has a boyfriend and she knows he's seeing someone. He wanted to know if I would be ok with it. We had a date scheduled and he's essentially breaking it to take her to dinner. He assured me that there's nothing between them but friendship and said if I wanted, I could go with him. Trying to be cool about it I told him to go with her to dinner and we would get together after. He stayed the night with me last night and we're going hiking together this afternoon, then we'll be together after his dinner. But I'm regretting telling him it's ok. I can't get it out of my mind. I guess, since they didn't really break up only because of demographics, I'm afraid the spark will still be there. I don't really think it's right that he even wants to go, but maybe I'm overreacting. What do you think?