That's a point, but I think it's likely because he owed her his own life. A debt from him personally, rather than from any God.I think she was owed, because she got to choose. If any 3 would do, Jaqen would have chosen them himself and fucked off. But she held him hostage to her choices to the point of choosing him. Obviously, she was owed.
At least that's my take on why she could pick. Either that or he was testing her for possible recruitment right from the start.