Probably not but I doubt she is in any way contractually obligated to Channel 5 unless there was any type of non-compete clause in her contract upon retirement. And if there was, it would probably be more tv-related than radio.
Then again, following her retirement last June, she came back in September to again co-host Channel 5's local coverage of the MDA Telethon and she may do that again this year so she wouldn't want to do anything to hurt that.