Which version of iChat ?
What sort of Buddy ?
And on what platform is the Buddy ?
If you and he are using AIM then go to your iChat Menu > Preferences > Accounts > Privacy tab and check you have him Allowed (either Allow All or he is in th Allowed Specific people List)
It may pay to change this setting to something else and back again to see if it changes anything.
Have you ever tested Parental Controls in a Text Mac User Account on your Mac using the same iChat Screen Name and this person's Screen Name ?
This will flag the Person at the Server end and show up in all your Logins.
If it is Jabber is he showing Off Line Buddies ?
If or when he is do you appear in his Off Line list as Waiting for Authorisation ?
If so, he should delete you and then re-add you when he knows you can answer (like when you are chatting)
This is about how often iChat send this Authorisation Request compared to other apps.
Please, if posting Logs, do not post any Log info after the line "Binary Images for iChat"
iMac 2.5Ghz 5i 2011 (Lion 10.7.2)
G4/1GhzDual MDD (Leopard 10.5.8)
MacBookPro 2Gb (Snow Leopard 10.6.8)
Mac OS X (10.6.8),
"Limit the Logs to the Bits above Binary Images." No, Seriously