I would never fault someone for a single accidental breeding.

On two occasions with two diffferent young Shep/Labs this same thing happened to me...not because I didn't take every possible precaution, but because they went into an early heat while waiting for their spay appointments...and others (not family members) let them out of confinement where they mated with other Shep/Labs...believe it or not! I mean, what are the odds of that!

I never intentionally allowed/caused breeding to occur, and had done everything within my power to prevent it. I kept my females in heat on short leads when I took them out, and went out first to make sure there were no interested male suitors camped nearby before either of them stepped foot out the door.
A mistake is a mistake, and they happen to perfectly wonderful careful and responsible people. These people sound as if they love their babies, which is all well and good and I am truly glad about that, but, it also sounds like they have intentionally bred her on previous occasions, and they know she takes off when outside...which sounds a bit (ahem) irresponsible when you know this and you know that your 4-lb Yorkie is unaltered and in heat...so I have to wonder why she was not on a lead accompanied by a human. I do not have any idea how large the Dachshund was, but this and the number of pups she carries would be my concerns because I have seen some pretty large Dachshunds myself, so I just hope everything goes okay for the little Yorkie welping more than 5 'Dorkie' pups. I had a good sized 'Doodle' years ago, but the mom was a Standard Poodle and the dad a Dachshund, and no problems welping. I have a friend with a 'Dorkie' now, and he is a pretty little guy, but I do not know if his dam was a Yorkie or a Dachshund...she rescued him because his previous owner died.
I look forward to an updated report, if you are able to get one, and, hey, photos of cute puppies would be nice too, if possible....hint...hint...
Take care!