The statistics probably are in her favor (
http://miscarriage.about.com/od/pregnancyafterloss/f/oddsheartbeat.htm), even with the one loss already, but you can never tell how an individual case will go.
Does she have irregular periods or does she know for sure when she ovulated?
I measured 2 weeks behind at my 9 week appointment. I thought that was a huge difference and - having had two early losses before - freaked out, but the doctor was not concerned. We had our 12 week (so 14 weeks LMP) appointment this past week and all is well. I think my cycle was still recovering from the previous miscarriage and that's why the dates were off.
Agree with PP about seeing if she can get another u/s before 12 weeks to avoid the wait and stress. Best of luck to you guys.