I, personally, wouldn't go because I would be uncomfortable with dp going to meet up with an ex. I know that neither of us would do anything inappropriate or anything, it's just a comfort thing.
If he didn't even want you to mention this man, it sounds like he would also be really uncomfortable with you meeting him for lunch. It sounds like you have the potential of really hurting your dh's feelings if you go behind his back to see the ex. JMO, take it or leave it.
If he didn't even want you to mention this man, it sounds like he would also be really uncomfortable with you meeting him for lunch. It sounds like you have the potential of really hurting your dh's feelings if you go behind his back to see the ex. JMO, take it or leave it.