The answer is little to none...at first. When the two friends are first reunited, Victor is so overcome at seeing Clerval, and so horrified by his creation, that he cannot bring himself to say anything. Note his reaction upon seeing Clerval:
I could not endure to think of, and far less to allude to, the occurrences of the preceding night. I walked with a quick pace, and we soon arrived at my college. I then reflected, and the thought made me shiver, that the creature whom I had left in my apartment might still be there, alive and walking about. I dreaded to behold this monster, but I feared still more that Henry should see him.
So Victor can't even think about what he's done, much less tell his friend. Even though he's afraid to return home and find the creature, he's much more concerned that Henry will see it. So he's certainly not willing to tell Henry of its existence and creation. However, later in the chapter Victor falls ill. During his fever, he reveals his dark secret.
The form of the monster on whom I had bestowed existence was forever before my eyes, and I raved incessantly concerning him. Doubtless my words surprised Henry; he at first believed them to be the wanderings of my disturbed imagination, but the pertinacity with which I continually recurred to the same subject persuaded him that my disorder indeed owed its origin to some uncommon and terrible event.
So Henry doesn't really believe Victor, but he knows it has to be mroe than just the fever talking. Victor repeats the same story too many times with such conviction that Henry is eventually convinced that something has happened...he just doesn't know what.