That all depends on which software each unit has in it Irishrover. The ‘line’ is the 5.310 version software: both units need to be running a version of software (not necessarily the same version) lower than 5.310 or both units need to be running a version of software (again not necessarily the same version) higher than 5.310.
It would be best that he updates both to the current software off the Humminbird web site before powering on with the InterLink module. He should also save the Nav Data (Waypoints, Tracks and Routes) off of each unit to his computer and than Delete the Nav Data from each unit before updating the software and only reinstall it after the InterLink is up and running.