I fly out of DCA on Sunday's all the time - I've never had an issue with parking at DCA. But, I guess it's better safe than sorry. One option, as they're driving up from RIC, would be to see if any local hotels have a Park & Fly rate where they could stay the night before. Not that you'll have issues, but it would suck for there to be a bad accident on I-95 or horrible traffic at the mixing bowl or near Potomac Mills...

With most Park & Fly rates, you get a week or two worth of parking also.