Frankly, I can't really see how Tom didn't know this, since it's somewhat canon that Diana doesn't lie all that well, and this is a huge false construction, and the fact that Tresser is much much better at deception than she is. (not a value judgement, but it's his job and his life, and he [used to be, anyways] very good at it)
However, that doesn't mean he doesn't love her, and if he accepted the fact that she didn't love him as much as he did her, I can see this playing out where they both get to keep their dignity.