Changing the Prop?

I'm going to help my buddie out while he is on vacation and get his prop right. I guess he installed a new one and put in on wrong and it is way off balance. I found a prop puller and he was talking about their is a groove in the shaft that has a piece of something that goes into it before the prop is put back on. Now I have not done this before but he said it's not to hard, ya and he did it wrong and now I'm going to try it. He mentioned that their is groves that the prop will only fit one way but not to forget this piece that goes between the prop and the shaft, and is supposed to give if the prop hits something so your don't kill your shaft and prop. Well he forgot this piece and the prop vibrates when you give it gas. Is this a do it yourself project or should I leave it to the boat shop? I have offered to detail his boat inside and out just to say thanks for the pulls this summer and thought I could also get this prop on right so he can use his boat right when he gets back and enjoy the fall riding. I have it for 12 days and think not only I can get it to shine but in working order also. Any advice is welcome please.