Create a full plex.tv user account for your partner.
Share your server with this new account.
Ask your friend to share his server with this new account as well.
If you have Plex pass, invite this account additionally into your Plex Home.
That way you get a “hybrid” account, which appears on the Switch User screen, inherits the right to use the Plex mobile apps for free from your Plex Pass and can be shared with by your Plex friends.