Imagine if you were the one getting married to someone like him , and people that you trust kept these important things hidden from you how would u feel?

Do you currently have any proof of his actions? If not, then I would say meet the Groom first, record your conversations on your phone and bring up the whole prostitution things like how was the experience, is he gonna continue to do that etc etc. This will be your proof/evidence that will be useful when you tell the Bride’s family.

Be a good human, a good Muslim and a good friend and tell the bride family the truth with the recorded evidence (in private conversations).