Pack, I get what you are saying about not wanting to be fake with her. However, I think you just continue to DB. You treat her with politeness. You are kind like you would be to a total stranger. But you do not over share with her, just like the cashier at the store.

Her: "Hi, how'd it go with them?"
You: "It went well. We did some fun things, I am sure they will tell you."
Her: "Ok I will have to discuss it with them, but they seem like they enjoyed it."
You: "Yes, they really did seem to have a good time."
Her: "Okay well, we have some fun things planned to, we are going to go here and do this."
You: "That sounds great! They should enjoy that. Hey, I have to get going, I have some place I need to be."

Then leave.

R2C likes to say "Never stop DBing!" I agree with him. You DB with her for the rest of your life. Not to save the marriage (since that is now beyond all of that), but to have a friendly, polite R with her but still keep her at arms length.