A friend is going to be needing to get from one of Melbourne's airports to the other. There doesn't seem to be any direct transfer. Is there a method (other than taxi) to get from one to the other, or will he need to Skybus into Southern Cross station and out again?
There is no direct transfer, Avalon gets only a few flights a day and nobody books connecting flights between the two on purpose.
If you're really pinching pennies, there are zero public transport alternatives for Avalon, but it's possible to take a train from Southern Cross to Broadmeadows and switch to an occasional bus for the price of a normal Zone 2 ticket. This takes approximately forever though, and Broadmeadows is not the most savoury place in Melbourne to hang out at night. Details on Wikivoyage.
Last and least, a taxi from Avalon to Tulla would be well north of $100, but given the cost of the bus this might actually make sense for a group of three or more.
Take taxi from Avalon Airport to Lara Railway Station and from there take V/ Line Regional train to Footscray Rail Station. From there take Metro train with direction Williamstown and stop at North Melbourne Railway Station, platform 5. Go to platform 2 and take Metro train with direction Craigieburn to Broadmeadows Station, platform 2. Then take Bus Route 901 that will drop you in front of Melbourne International Airport / Tullamarine