rfs, if you already discussed this with the Chosson and he said that his RY told him he doesn’t have to tell about his past but yet you have seen him do the same things that were supposed to “stay” in his past and yet is still in his present then maybe you should speak to the Kallah’s Rav and have him speak to the chosson’s RY. Let the two of them figure out what to do in this situation. The Kallah’s Rav obviously will want and need to do what is best for her and her family and will take this seriously, knowing how the Kallah feels about what she is looking for in a mate. He will know how to approach the RY and then together will figure out whether the Chosson should reveal to the Kallah his true personality or the Rav should tell the Kallah and her family that the shidduch should not continue.
You should definitely NOT be the one to tell the Kallah anything at this point.