The first question is whether the user can login to OSN directly (i.e. into the stand-alone OSN web UI). If they can login, then it's possible there's a problem with their provisioning. That can be cleared up by an Administrator through the Admin UI (they just need to resync the users with FA). If they cannot login, then it's possible the user wasn't provisioned at all, in which case an Administrator should check their account settings in LDAP (and possibly resync users with FA again, afterwards).
Note that Outside Users do not have access to social objects. So if this is a partner (as opposed to an employee), they will not be able to see some conversations related to some business objects. I don't think that's the case here, given the information you provided, but I mention it for completeness.
Hope that helps,