An 18 is intended for 5-6 to 6-0, a 19 is for 5-9 to 6-3. So it depends whether he likes a smallish frame or a bigger one.
I don't think it follows that a 6-0 with extremely long legs should go for a very big frame. A very big frame will have a very long top tube and a 6-0 with extremely long legs will have a short upper body, and therefore find the long top tube a stretch. Best posture for somebody with long legs and short upper body could come with a short top tube and very long seat post. But a Kona is not a good make for that shape of person anyway.