That PrC would require you to, at the very least, dip a spellcasting class, blow feats on Point Blank Shot, Craft Magic Arms and Armor, and Practiced Spellcaster (the last one can be skipped, if you want to spend four more levels on that casting class). It offers full casting, yeah, but you've already lost six caster levels already.
As for the Swordsage build, it's mostly maneuver selection. Pick up some stuff from Black Rain for damage, add in some Shadow Hand for movement and Assassin's Stance, and you should be golden.
Honestly, I'd see if the DM is amenable to him retraining into a pure Swordsage. The base class you linked isn't that great, to be honest--it's basically the Fighter without bonus feats, but you get:
Two Weapon Fighting (which you would have taken anyway),
Quick Draw (which could easily be replicated with a Least Crystal of Return for 300 gp),
Rapid Reload (and depending on the gun rules, this is needlessly painful),
A bonus on called shots/sneak attack damage that's too little too late,
Swift Reflexes (which is Uncanny Dodge's less attractive sister),
Eagle Eye (which doesn't even make sense as written--it sounds like it's talking about threat range, but then the example makes it into some range increment thing),
Reloader Shot (seriously, anyone who ever writes a gun class ever seems to hate firearms, because they always have a marginally higher base damage but take the rest of the combat to reload),
and Leadership with doubled followers (and sadly, your cohort has the very real potential to be stronger than you).
Um, no thanks. Just go pure Swordsage, see if the DM will let your friend swap his mental stats to Int 11, Wis 14, Cha 13 (you actually gain skill points this way), focus your maneuver selections on Black Rain, Shadow Hand, and the like, and you should be fine.