Well I have to say my personal philosophy runs along the saying, "old enough to bleed, old enough to breed."
My main issue is that both of them are emotionally damaged individuals, and at least he is not thinking it through. He is putting his family and career at risk, and he should know better as he has lived for a long time. Now if he was making a conscious decision to take the risk to go after her it would be one thing, but he does not seem to be doing so, and that is irresponsible.
The girl on the other hand I am not certain if she has thought it through or not, but she is in a horrible situation and this might help her get out of it. Marrying out of the family or eloping is always one way to escape. However, she is risking wrecking his life and that is irresponsible, but she is not old enough to be expected to know better so it is to be expected.