I guess your WW should have thought about the consequences of her actions before she decided to commit adultery.

If you fear the repercussions of exposure, from your WW, you have a poor chance of saving your M.

Contact the OM's BW, your WW's family and any other significant others who have influence over WW.
The OM's BW will be your best source of ending this right now, you said that he will not leave his family due to these circumstances of the army.
Would he not drop her like a hot potato if he thought that this A might be founded out???

M'd 22 years
D-Day 08/08 LTA