Wow, that seems kinda late, but I'm not an expert in this area. Mine are 2 and 4 and are potty trained at night which makes me think that this poor lady might need to see a urologist.
What time of night does she have the accident? If she can stay dry through most of the night, but can't make it to morning, perhaps you should go in and wake her to try to "catch it" before it happens? Then let her go back to sleep.
If she has the accident(s) in the middle of the night, perhaps she has a sleep condition that doesn't allow her to wake up?
If anything, I'd do a potty stakeout to gather some data to present to the dr! I'd try to figure out the times during the night it happens, so my dr. would have more to go on. (Yeah, I'm that mom! lol!)
Best of luck to you - let us know how it goes! :)