Hi,
I just did this using Bonjour between a Snow Leopard Computer (iChat 5) and Messages in Mountain Lion (version 7)
I started from the Snow Leopard Computer and Offered to "Share My Screen" from there.
The MacBook Pro (Snow Leopard) Screen cam up on the iMac (Mountion Lion).
I could see the cursor on both Screens as I moved around the pic/screen on my iMac the cursor moved on the MacBook Pro.
I tried bringing my own Screen to the front (no Problems) and then mousing over the reduced MacBook Pro screen. It seems it is isolated from the Cursor (As it should be) at this point and did not appear on the MacBook Pro.
How many physical Screens/Displays does the Buddy have ?
Any chance your cursor is so far off to one side that it is not on the view you can see ?
This might also depend on how the Buddy has any external Screens set up.
Most run them left and right but some have them one above the over.
It may also depend if the display is actually On. I once used a second Display/Monitor/Screen and had it turned Off but System Preferences identified it as being "there" and in an iChat Screen Sharing it distorted the view. Very wide letter boxed pic.
10:40 PM Tuesday; July 31, 2012
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