Personally, at 6', I'd definitely at least try the 58cm first(assuming you don't find yourself a suitable 56). I'm 5'7" with a slightly shorter inseam than him and ride a 54-55cm. I think he'd either be cramped or have his bars well below the seat on a 54cm. ...Unless that's the position he's comfortable in over long distances.

Personally, I think you should focus more on top-tube length than seat-tube height. Measure from seat-post to hand position on his current bike(assuming it's a good touring height for him). Then try to match that on the other frames. Height can be adjusted fairly easily once you find the frame you want to use.