I honeslty would'nt GIVE the OW a 48 hour warning. That gives her 48 hours to delete her emails, texts, phone bills, etc and she can deny it and so can your H.
I would contact the OW H immediately and BEFORE you tell your H you know.
If you don't have any hard evidence other than your own witness testimony they can both deny it and its the OW word against yours...
They could make you out to look like a paranoid wife and laugh it off...
You need EVIDENCE and don't give them any warning... You will likely regret that
Your best ally in this battle is the OW H and if you don't tell HIM asap he isnt' going to trust you or want to work with you... he WILL believe his wife befroe you most likley.. Unless you have hard evidence.
I would team up with the H to collect evidence asap. If he doens't believe you tell him to collect the evidence himself but not to confront her because then she can clean up her tracks...
WHy would you WARN them? It's just giong to make it all the harder for you later on.
I know you think he isn't going to lie, but he already has lied to you.... He may very well deny the whole thing... affairs are VERY addictive.. make sure you have PROOF before you confront him...
