You doing HTR/RTR only or does he want an ETR as well?

If it's just RTR/HTR, I personally enjoy pure class.  (Mainly because I don't want anything standing between me and Blade Barrier.)  Max INT, just enough DEX (plus tome) to pick up Improved Precise Shot and the rest into CON.

I tend to play Arcanotech with Harper as my backup enhancement chain.  Just spam the electric SLA's and in the 2 second window at the end, plink away with a volley of crossbow bolts.

Have the Holy trinity of metamagics by level 9: Maximize, Empower, Quicken.
Ranged feats.

Enhancements:
Levels 1-3: make a beeline to Strategic Combat 2 in Harper.  Make unmeta'ed use of Static Shock and Lightning Sphere.
Levels 4-6: Invest into Arcanotech for Wand/Scroll Mastery, the SLAs and Electric spell power.  Start replacing Static Shock and Lightning Sphere spells with the SLA versions (meta'ed up the wazoo) on your hotbars.
Level 7: Reset Harper and only pick up enough to get Know the Angles 3.  Invest recovered points into Arcanotech for 3rd core, More SLA action, Electric Spellpower and crits, INT.  Start giggling at how little SP you use for the nifty CC you get.
Levels 8-10: More INT, More crit, more spellpower from Arcanotech.
Level 11: Wait for it....
Level 12: Pick up Lightning Bolt SLA and Lightning Motes SLA.
Level 13: Grab Arcane Mechanism.
Level 14-17: Invest in Harper. More INT, Versatile Adepts and one rank of Highly skilled.   You can also pick up a few low hanging enhancements from race/Battle Engineer.  Just keep in mind that Battle Engineer is kinda terrible for repeating crossbow users.
Level 18: Pick up Critical Admixture.

Spells to keep memmed:
Level 6: Flame Turret
Level 7: Insightful Damage
Level 9: Armor of Speed
Level 12: Prismatic Strike
Level 15: Blade Barrier
Level 16: Reconstruct

This will all change in U36.

If you're going to 30, he might want to take a look at the Electric Critzilla.

Don't forget to mute sound. Most important element of any repeater build.

I even hate that sound in text.

Mine sounds like: