If your friend is using a Guide, he should use the Guides equipment. All the lines and knots will be properly tied, he will also know the proper flies to use in the area your fishing.
Being a Veteran Tarpon fishermen the line I use 90% of the time on my 12 wt. Is S/A WF-F Tarpon taper. He may also need a WF Intermediate if there is a lot of weed. The line I would recomend is a Cortland 444SL salt water intermediate for the second line he can use the Cortland for Striper fishing.
PS: If he is fishing for Tarpon on a 9 wt, He is after Babies, most of the Guides are using 12 Wt's.
Please have him e-mail me if I can be of any help.