From personal experience, since I had to play in 'horn' keys all the time at my previous church.

It's my opinion that the chosen capo position and open chord shapes depend on the feel of the song, which would be preferred. For many modern rock and worship songs, the sound of add2 and sus4 chords is almost a given and open Cadd2 Gsus4 are easy to 'anchor' and move back and forth. For more gospel-oriented songs, where these chords are less common but perhaps dominant 7 chords are called for, then open E7, C7, A7 shapes are more comfortable. It really depends on the player's past experience.

As a result, I wouldn't sweat this too much - your friend Roger, if he's used to using a capo, likely already knows what to do. Print out the chord sheet in the actual key you are playing in, and let Roger work out what his preferred capo position will be.

Have this conversation with Roger and see what he thinks. Guitar players that sit in with bands are pretty used to transposing/capoing.

-Scott

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