If you are concerned for your husband's safety then you SHOULD tell the OWH. If YOU tell the guy its safer than OWH CATCHING YOUR HUSBAND with his WIFE.
Believe me the safest thing to do is get this out in the open FAST. The problem is affairs are highly addictive and H may just deny its happening and YOU will have TWICE as hard a time getting proof...
Right NOW they don't know you KNOW, so they are going to be sloppy... ONCE you confront they will be a LOT more careful.
You need to get OWH on your side. If you can get a photo of the affair happening that's best... Hire a PI if you can afford it.
I honeslty would tell the OWH safely rather than risking your husband getting caught in the ACT...
Do some detective work on teh OWH... find out what kind of person he is if you can...
Go meet him and pretend its an accident.. if he's a dentist, make an appointment or something.
Its hard to offer more advice since we don't know your H, but many will deny it OR will admit it, but refuse to stop...
Michel's DR book explains this part well.
Not Just Friend's is by far the best text in my opinion.
But i would NOT let this go on much longer.. but you need OWH on your side here...
If you think just telling your H outright willg et him to stop and that's it then do it, but practice has shown this isn't the response very often...
More often its
1. Denial OR... 2. Admitting the affair but refusing to stop
YOU COULD Confront the OW in her HOME... take your daughter with you for some influence... if they see you have kids you will get taken far more seriously.
Tell her and OWH that you love your husband and youw ould just like this to stop.. quickly and quietly...
Going THAT route may have more impact that confronting your H.
If you can get some emails and print them up to take with you do that... get some material proof or they will possibly deny this is happening and you will feel worse than you do now.. You need to proove it somehow.
