itc reverse or not

You cannot take ITC when he was composite dealer, 'reverse the ITC' or 'add to output tax liability and make the payment of tax' along with interest @ 24% p.a.

Chetan ji if you want to tell us that you are purchaser then the other person will be seller and not buyer, A dealer under composition scheme can not issue tax invoice nor he can collect GST from their customers. As you have been cheated by that person you can take legal action for the same. Regarding ITC, you have to reverse the amount of ITC claimed as it will never be reflected in your Electronic credit ledger.