Well, because /dark is very active mitigation, then I'd suggest something more passive, or perhaps reactive. I'd think that Traps or FF would go well. With FF, there is mez protection and decent safety for the zombie/dark to debuff with, and it'd generally shield the pets better. Traps would give you a few more things to do as well though. Thinking about it, just about every secondary would go well. Just go with whatever you'd fancy.
To be honest I think any combo works well (asuming neither of you have that horrible Trick Arrow set) as you have an army to hand. My Mastermind is Necro/Poison and my mates is Thugs/Traps. We team all the time and i dont think it matters much due to the strength in numbers.
Heya!.
I've been reading throught around 30 pages and I've seen there is no bad mastermind.
I've tried a thugs/dark MM and i like it very much, but i also want a MM who can go well with my friend's zombie/dark.
Any suggestions are welcome.
thxs in advance.