1.Nf3 d5 2. Nc3 Nf6 3.d3 c5 looks good to me with Nc6 and maybe e5 in the next few moves if you can get away with it. No pawns by white in the center gives black a good chance to grab the center and space. White is playing passive so take the reins!
Could Not Find Anything On This Opening

Be careful when playing c5, d5, e5... U could tranpose to something reversed.
Here, it can tranpose to a Nimzo line (reversed a i said) ater 1.Nf3 d5 2.Nc3 c5 3.e3.
White plan is usually to achieve e4 with Bb5, Qe2, d3, e4, playing Nimzo Indian style.
U may wonder why e3 and not d3 first to play e4 in 1 move?
Well, why not playing d4 with black?
So 1.Nf3 d5 2.Nc3 Nf6 and on all moves from white not preventing d4, just play it.
For instance 3.d3 d4 4.Ne4 (where else?) Nxe4 and u already took some advantage in my opinion, u can also play 4...Bf5 to speed up development, not fearing Nxf6, u would take with e pawn once again speeding up your development.
On 3.e3, why not preventing Bb5 before playing c5, so i propose a6, then c5, if u really want to play c5. Bb5 is good only if black play c5...
If white cannot play Bb, this bishop has no active sqaure available, he will go probably on e2, then c5, threatening d4 and taking centre...
U can also notice that e3 will block Bc1, so this Nc3 move brings some problems for white, because this Nc3 is not in a good square and u can benefit from it...
Thanks guys! Hopefully the next time we play I will know what to do, he is almost double my rating so the advice is definately needed.

The Black Knights' Tango (aka Mexican Defense) starts with 1...Nf6 and 2...Nc6, so it appears thaht your opponent used the White Knights' Tango (aka Mexican Defense, colors-reversed). The Black Knights' Tango aims to develop flexibly, and strike the center with either ...e5 or ...e6 followed by ...d5. Transposition to various mainstrean openings are possible. Presumably white could adapt the same goals in the colors-reversed form.
So I recently started playing with a co-worker and he uses this opening a lot, 2 out of the 3 games we played.
I play as black here, and I'm just wondering what the best plan is against this opening. His next move is d3 in this situation. He often looks to trade off his knights early.