Aggressively work to bust the affair with everything you've got: confrontation of the cheating spouse, exposure to key family and other influencers, gathering intel to confirm the truth of what's going on, strong legal stance, etc.
Allen A posts prolifically over on the Infidelity forum. Just a casual reading of a half a dozen or so of his posts will give you a feel for the approach, as he's very methodical and consistent.
