Honestly, if you're not superclose with the daughter, then I'd go to the conference. I know my parents invited some of their friends who (no offense to them) I didn't really care if they showed up or not simply because I wasn't close to them. It'd save them money on the reception as well. If you and the mom are still very tight, though, it could be a different story. But as long as the friend is your average friend, I'd say they wouldn't be too offended if you had a conference to go to. As long as you hadn't already RSVP'd and been paid for, they should be fine
ETA: Sorry for the intrusion, I didn't realize this was in the ADPi forum