If you don't use the tent will you be renting a cabin with her (eek!) or using other camping equipment?
If you have the means to get a different tent, I would just go about setting up your stuff when you get to the campsite. Then, when she asks where her tent is say "You know, when we were airing out the tent before we packed it up in the car we noticed a couple of holes in it. We were really bummed that we couldn't use that great old tent, but we got this one at Target (or from our neighbor Bob or whatever) on the drive up, and it looks pretty weather resistant"
IF however, she is going to continue babbling on about her wonderful old tent every moment from now until your trip, you might mention the holes a bit sooner. It might embarrass her a bit, if it's brought up in a crowd, but there's no point in letting her go on and on about it either.