1) they may be playing the international version, which makes it very easy to change sphere grid paths

2) they take one person's sphere grid for a few SLVLs, then go immediately to another.
That would be how I made kimarhi a white mage. He has to go through rikku's grid, but right back from teh beginning, there's yuna's grid.

A summary of kimarhi's possibilities:

1) Straight up, a level 1 lock to the beginning of tidus' grid
1-a) Beginning of Auron's grid, take the level 2 lock right past Haste
2) Down right, a level 1 lock to Wakka's beginning
2-a) Middle of Lulu's grid at Demi, a level 2 lock at Silence Attack
3)Straight down, level 1 lock to the beginning of lulu's grid
3-a) End of Rikku's grid, at Bribe, a level 3 lock at Fira
4) Down left, a level 1 lock to Rikku's beginning
4-a) Middle of Yuna's grid, at Dispel, a level 2 lock at Use
4-b) End of Tidus' grid, at Quick Hit, level 3 lock back a ways from where you came in Yuna's grid, right around Shell
5) Top left, beginning of Yuna's grid, level 1 lock
5-a) middle of Tidus' grid, level 2 lock, at Delay Buster

The best path to take is Yuna's. You don't need to backtrack, and you won't be using kimarhi to do damage until you get Holy. There's more important things.

You'll want to do 4-a. Go into the beginning of Rikku's path, get Steal, and Use. Use a lvl 2 ks to get into Yuna's sphere at Dispel. This spell alone is worth taking this path. You may want to backtrack for Shell, Reflect, and Protect, but don't worry about Cura. (if you have a lvl3 ks you can get Quick Hit, but you won't be needing it, since you won't be hitting)

Kimarhi's role in your party should be a secondary thealer. That's what rikku is, and it's insanely useful.

You can steal double as much, getting tons of awesome items (make sure you get lots of light curtains fromt eh iron giants in the thunderplains, btw) You can use al bhed potions twice as often (1000+ hp and gets rid of the most common status ailments? awesome!).

And most important? You can get rid of Evrae's hastedness in that battle. You NEED that dispel, it'll just make it so much easier. Not only can he not attack as quickly, but he'll waste extra turns casting it back on himself. And since he's immune to slow[ga] this is the ONLY way to get rid of it.

If you're worried about a certain battle on mt. gagazet, don't. Even if you were worried, you ought to have Kimarhi learning Holy by then, so even if you don't win with blue magic, you have a spell that blasts at about 7000dmg (Since kimarhi jumps in yuna's grid a lot later than yuna does, he learns holy much sooner, and far before zanarkand)

And speaking of that place, you have two mages that can blast the boss of that place with the spell that does the most damage to it