If you know neither usernamenor password from his mail, you first need to know the username. For this you can ask him about it under the pretext to send any important document. Once you have at the hands of his mail, you can proceed to the second part of the plan.
Register a new account on the social network, which often deals your friend. The best option is to register under the guise of a female person, because it will be easier to find the information that interests you.
Within a few weeks chat with this account on the network, try to communicate as much as possible and often. Add new friends, update statuses, add photos and videos, in short, keep an active life in social networks. This is necessary in order not to arouse suspicion from your friend in the future.
Add your friend's friends, pretend he's wondering. Keep a distance, but genuinely be interested in his Hobbies. This will stimulate reciprocal interest. Maintain easy, non-Intrusive communication.
In order to change the password from the mailbox, you will need to know the answer to the secret question. Secret question and answer, you specified during registration of mail, people often point it out, without thinking about the consequences. In an unobtrusive manner move one of the dialogs to the subject, which deals with secret question and ask a question similar to secret. Getting the answer and making sure that the answer fits to the question, change the password. After that abort the dialogue and will delete the page.
Go to the social network from your account and tell other what and how you did, then give him access to the mailbox.