Sometimes a tooth that has been knocked out in an accident can be put back in and it "takes root" so-to-speak. So, given this, I believe that it is possible for the molar to reattach, I've just never heard of this before. Being that it was a mistake I have a feeling the dentist will pay for the bridge, in case the molar doesn't take to be replanted.
The condition is similar to transplantation. Root canal treatment of second molar is warranted.the transplanted tooth generally will be fixed again as time goes by.