As you know AIM will stop as a service on the 17th December (2017 for later readers).
Yahoo that was added in iChat 6 has also ended by Sierra as Yahoo ended the previous services and wrote the new program from the ground up and Apple have not added it back to messages.
This leave the iMessage abilities and the possible adding of a Jabber account.
Google run a Jabber server and your Google ID is a valid Jabber ID for this server.
In High Sierra you have to add the account in System Preferences > Internet Accounts under Other > Messages then choose Jabber as the type, enter your Google ID and password then chose to Automatically find Server and Port.
At their end they would need an app that can also use Jabber IDs
If they were using AIM for PC this will not work as even though both AIM and Google offered behind the scene links to other messaging services the AIM server will be shut down.
They would need a dedicated Jabber app or one such as Trillian that can join several services.
7:56 pm Sunday; December 17, 2017
iMac 2.5Ghz i5 2011 (Sierra)
G4/1GhzDual MDD (Leopard 10.5.8)
MacBookPro 2Gb (Snow Leopard 10.6.8)
Mac OS X (10.6.8),
iPhone 6 iOS 11.x and an iPad (2)