Cut out something from a video ( make a green screen )

I have a small video saved from Instagram, there is a famous artist that appears on the video while one guy is filming himself. So what I want is someone to cut out everything except the famous guy that I want. (the video is about 3 sec) I want this video so I can use it as a mask/layer over an other video later. So if I can use in in Final Cut or just make a green screen video that I later can cut out on Final cut.