Ray wants to silence his former partner for snitching to the police. You must firebomb his apartment and finish him off when he emerges with his police escort.
Follow your radar to the witness protection house in central Staunton Island. Ray has given you a dozen grenades so toss one of them through the open window.
When that happens, a nearby garage will open and your target will drive off. He has several police officers on foot who are heavily armed, so don't stick around unless you want to keep playing with grenades. Ray's partner doesn't have anything up his sleeve at this point, so run his car off the road and blow him up or kill him when he flees on foot.