Well, "Donnie Brasco" was an FBI agent, not an actual criminal. So the rules worked a little differently, and in truth, Joe Pistone (his real name) was in witness protection for some time with a $500k bounty on his head. To this day, he disguises himself when he travels.
I wouldn't count on any kind of police protection unless such were the terms of his plea agreement. If the people he gave up were well-connected underground types, I'd suggest he consider relocating and possibly changing his name. On the other hand, if they were truly out to get him they'd likely have made a move on him in prison, where those kinds of criminal organizations usually have a number of loyal subjects and no one talks. Dealing him damage outside prison walls is actually riskier.
He did the right thing cooperating with the police. Unfortunately, one of the risks of running drugs is that you can be put in a position to "disappoint" some very unstable, violent people. His focus should be to find an honest line of work and live a life off the RADAR for awhile.