Weak crown of a tooth

My wife had a root canal, and the dentist covered the tooth with a temporary crown, intended for the next 20 days. The dentist said that he glued it on so well that it will could hold for a long time. After a few days my wife felt that the crown cracked. The next day she was supposed to immerse in a mikveh (a ritual bath). We asked a rabbi, what we should do. He said that if the crown is glued on well, it does not constitute a barrier (“chatzitzah”). So, my wife immersed. The next day, as she was flossing her teeth with dental floss, the crown came off. It’s important to note that the day before, while preparing herself for immersion, my wife also flossed her teeth, and nothing happened to the crown. The question is, if the crown came off, does this mean that the day before it was already loose, and it did constitute a barrier (“chatzitzah”) already then, or maybe, if it was still attached to the tooth, it did not constitute a barrier?

Dear …!

If at the time of the immersion the crown was attached to the tooth, it did not constitute a barrier, and the immersion was adequate.