Disappointment and Dilemma

We were supposed to go to Quanah this weekend to see Eric's parents. This was a pretty important trip (some of you know why). However, when Eric called last night to let them know we were coming his mom informed him that she and a friend of hers are going to Dallas this weekend to shop and to go to Canton. While I am a big fan of Canton, and I wish I was going with her, I really wish there was a way we could get her to cancel the trip. I told Eric he should call the friend and explain to her why we really need to come see them this weekend and have her make up some excuse for needing to reschedule the trip. Or I thought maybe we could call his aunt (who lives in Dallas and will be joining them in their shopping ventures) and try to get her to convince them to cancel and come some other time. But we didn't do either of those things. So now our trip will have to be rescheduled. I guess it's not really that big of a deal, but I know when his mom finds out why we wanted to come this weekend she will wish we would have. I don't know what to do. Any ideas?