I have several problems with what they did:
First, I personally think it's unethical to conduct the negotiations for an alliance outside of game chat and that's pretty clearly what they did in this case. It's not strictly against the letter of the law, but in my opinion it violates the spirit of rule number 2 ("no secret alliances"). People have different opinions about this, but I think it warrants you complaining about it in game chat and if necessary leaving neutral or negative feedback for them (depending on how strongly you felt about it).
Second, I don't know much about escalating games, but it doesn't appear that you are necessarily the dominant player. While I have no problem whatsoever with alliances in a three-player game, it's usually the case of the two weakest players forming a non-aggression pact against the dominant player. That doesn't seem to be the case here. I would point that out in game chat and if necessary leave neutral or negative feedback for them.
Third, I find it somewhat difficult to believe this alliance has no terms. Again, I guess this isn't strictly against the letter of the law, but in my opinion it violates the spirit of rule number 2 ("no secret alliances"). I would ask in game chat what the terms of the agreement are. How long does it last, what's the termination clause? Is it solely a non-aggression pact or are they coordinating attacks? I would ask these questions and if I didn't get satisfactory answers I would consider leaving neutral or negative feedback for them.