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.
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.