My ex attempted the same. I had to simply follow it without deviation until she realized there was nothing she could do during my parenting time.
However, it has been 3 years since your friend has seen child. What type of effect do you think there would be should he walz right up to the door and say "come on, it's my weekend?".
Your friend made a choice to simply quit and for that I am sorry. The system places the non-custodial parent in a sub-class with no rights when it comes to child. They use the "best interest of child" to justify this.
My suggestion would be to have friend ease back into the parenting time-perhaps no overnights at first, maybe day time to a movie, park, library, book store, school lunch, etc etc. Then return child in the evening. Once a comfortable relationship is RE-established then go to overnights. If mom continues to block or interfere then each time file a police report. This will bolster a case and help establish the possibility of PAS.
If she is leaving threatening calls then record conversations and don't tell her (know your laws in your state).
Unfortunately it is what it is-your friend has missed out on a lot. It's never too late to re-connect.