Passes are sold to a specific person and cannot be transferred or used by someone else as the details will not match.

Depending on the specifics of the pass it may or may not be possible to exchange it, refer to whatever conditions were given when it was issued.

If this is a pass issued under the DiscoverEU scheme then your friend needs to contact them to rescind the pass so it can be issued to someone else, it is not his to pass on to you.

Please contact customer services with the details of the pass and they will inform you of the options available to you and your friend. https://eurail.zendesk.com/hc/en-001/requests/new