Hi,
Messages can join several services.
1) iMessages is the Apple one for text Chats to iOS devices or Macs (Messages) using Apple IDs and iPhone Numbers.
2) It can join AIM with an AIM name or an AIM Valid Screen Name such as Apple IDs ending in @me.com and @icloud .com issued from iCloud.
This does mean you can set up your Apple ID twice in the Accounts section of the Messages Preferences.
Once for iMessages and once for AIM.
3) It can join the Jabber service.
This is a collection of servers that issue Jabber IDs that can then be interlinked.
Google run a Jabber server called GoogleTalk.
You have to have a Google Mail ID and enable Talk on your Google Settings.
Google doesn't conform to the standard naming of the server which is why it has a separate entry in the Add Account pane.
Facebook also run a Jabber server and you have to enable "Chat" on your Facebook setting (at Facebook)
There are many other Jabber servers.
You can add Names from one Jabber server to the Buddy list of another (So a Facebook Buddy list can have Googletalk Buddies in it for example)
To be clear again.
iMessages can only "talk" to other iMessages accounts that have been registered (Apple IDs and iPhone Numbers. (Talk in this case is Text chat)
AIM and AIM Valid Names can only "talk" to other AIM and AIM valid Names. (Talk is Video, Text and Audio Only chats plus Screen Sharing)
Similarly Jabber is Jabber to Jabber IDs (Talk is the same as AIM)
So in the Contacts App his Google ID should be listed as Jabber not AIM
You need to get some form of Jabber ID to use in Messages if you don't already have one.
Or he needs to get an AIM (Valid) ID.
NOTES
Of course his gmail email ID may also be the one he registered for his Apple ID.
It may work in Messages if he is using Mountain Lion as an iMessages ID
On iChat 6 and below he can only use it as a Jabber/GoogleTalk ID.
If this Google Buddy happens to be using a Web Browser to Login then you will only be able to Text Chat.
Same if he is on a PC and using the PC app called GoogleTalk (Different Protocols for both options)
10:05 PM Monday; February 18, 2013
Please, if posting Logs, do not post any Log info after the line "Binary Images for iChat"
iMac 2.5Ghz 5i 2011 (Mountain Lion 10.8.2)
G4/1GhzDual MDD (Leopard 10.5.8)
MacBookPro 2Gb (Snow Leopard 10.6.8)
Mac OS X (10.6.8),
Couple of iPhones and an iPad
"Limit the Logs to the Bits above Binary Images." No, Seriously
