Is it wrong for me 2 be going out wit my friends EX?

okay so ive been txting my friends ex bf but i really dont consider her my FRIEND because she lied 2 me n him so she lied 2 both of us at the same time anyways last nite he asked me out and i said yes but then he brought up who was gunna tell her that we were going out but like she still likes him and she is really jealouse and also a lier but i was telling him that i would tell him but then he said that it really didnt matter because it wanst her relationship it was ours and he also made a point the so is it wrong for me to be going out wit him and should i also tell her or let her find out ????