What about the security question recovery method? Is that method of recovery available yet?
If not then you should have to contact the support and I don't think it is really possible to recover if you lost the number so just get the same number by providing your ID then recover your gmail.
It is possible.
He only needs to talk with the support and explain the scenario and what has happened to his account. If the details he will provide or if the support asks certain questions related to his email and he's able to answer and provide the accurate answer, they will spare him.