Defender advice would be easier to get in Defender forum =x
I was more looking for blaster advice, for this pairing. As he's new to the game he has alot to learn, and I dont' really know what to teach as I've never teamed on my fire/fire and never played a defender.
A Dark/Sonic can make anything look good. Grab tar patch, fearsome stare, howling twilight and darkest night along with your heal and it's pretty easy. The only thing that you may wish is having more AoEs. With tar patch's -res and the -res provided by your sonic blasts, stuff dies pretty fast.
Gaaah fxxk it. I typed all how/what to do sxxts, and clicked Continue button.
Then the site just logged me out, and that 2 letter pages are all gone.
Fxxk it. I'll pass.
Learn him to take a better primary, Archery 
Electric Blast trades some damage for safety, in the form of controls via End drain and holds.
With a Dark Defender around you aren't going to need that safety, so I'd advise your friend to NOT slot for End drain. Treat Short Circuit as a high accuracy damage attack, so six slot it for damage, recharge and a little bit of accuracy, maybe even squeeze a damage proc in there. Do the same for Ball lightning, but put more accuracy in because it doesnt have an acc bonus like Short Circuit.
Take the punches from the secondary and use them well - damage and recharge again to get as high a damage output as possible.
Aim and Build Up are required, as is Hasten to get those two self-buffs and the AoEs recycling as fast as possible.
Dark + Blaster works very well, but works best with a blaster's blaster like a Fire Blaster. So my main advice is to make your Elec Blaster concentrate on damage output as much as possible.
For your own power choices, my main advice is to avoid Shockwave,a s this will knock enemies out of your Tar Patch.
Get Howl though, for the -Res it provides on a group. Siren's Song is probably not necessary since you have Fearsome Stare.
So per-spawn tactics:
You lay down Tar Patch, Fearsome Stare them, Howl as your buddy closes to melee range and unleashes Short Circuit and Ball Lightning to kill the minions, he then cleans up the bosses and lieutenants with his electrified fists.
Next Group! 
Use Darkest Night and Twilight Grasp as required.
Thanks for the advice, I'll pass on the info. I realize this isn't the most "ideal" pair, but we're not trying to be ideal, just effective and fun.
I'm about to duo as a dark/sonic def with an irl friend of mine who's new to the game playing an elec/elec. As this is my first def, and my fire/fire is my only blaster I was looking for any advice on how to pair this combo up effectively. Thanks