If I had to guess, assuming she's not wearing any "shorts" under her shorts (pretty much a no-no), I'd say that those first rides are all going to be uncomfortable. However, it should be her sit-bones that are sore and any discomfort in the 'tweener' region would more likely be a saddle / riding position issue, e.g., saddle too high, saddle nose too high / not level or being too stretched-out (possible if she's short, but not likely if she's of average height). But, then again, I'm not built that way so these are merely suggestions based on what I've learned with Debbie and working out fit/comfort issues with other teams.
As for shorts, I guess if they were really awful or not true cycling shorts they could be part of the problem. There is some variability in cycling shorts and it's often times a trial and error thing to find the right shorts for a given rider's anatomy. Saddles are the same way and about as expensive.
Debbie's been very happy with her Koulias Zaard shorts for about 7 years, but I think they went out of business. REI's body suits were good at first, but the edging around the synthetic chamois wore thin in about a season and that gave way to some nasty chaffing. She also has two very nice body suits from Bellwether, circa 1999 but they too have ceased production, at least of this model/cut. I also picked up some Descente body suits for Debbie, but the legs are too long for her tastes. It goes without saying, somehow Debbie's Zaard and Bellweather shorts never seem to wear out. This is in stark contrast to me as I'll wear out Performance house-brand and Giordana bibs in a season, Pearl Izumi last about 1.5 seasons, Louis Garneau are good for 2 seasons and Santani bibs are "so far so good" after 1 season; time will tell.