In this context, "tipped off" means that he was given some privileged information; someone who knows some behind-the-scenes secrets shared the information with him.
For example: You are in a restaurant and tell the waitress that you'd like the daily fresh fish entree. The waitress leans in and tells you confidentially: "I'll bring you the fish if that's what you want, but I thought I'd let you know that the chef tipped me off - the delivery truck hasn't been here for two days, so that 'fresh' fish has been in the freezer for a while."
Another example: You are a prison inmate and have been planning an escape over the course of several months. The afternoon prior to the night you are ready to set your plan in action another inmate whispers to you during meal time "Greasy Thumb Eddie tipped me off - he works in the warden's office and he overheard that there is going to be a state inspection of the prison first thing tomorrow. That means that tonight the place will be filled with folks cleaning and fixing things all night long."
Interested in Language