Seller unable to remotely erase apple ID

I bought a phone from someone who forgot to do factory reset on it. The phone asked for password, I reached the seller, he said he will remove it from his icloud account. Truly the phone accepts Sim and can make calls and access the Internet but I cannot do factory reset to set up my icloud because its still asking for the password. The seller is willing to assist but doesn't seem to know what else to do, and we are presently countries apart. Please the seller needs a guide on what next to do to remove his ID from the phone.