I am so terribly sorry that you are having this nightmare become part of your reality.
Please, please, take care of yourself and your little ones, find someone you can trust to lean on, outside of your home. An IC can be the perfect place to unload, and say the things you don't feel you can say to anyone else.
Now, as far as taking care of yourself... this is another painful part of being the BS, please make an appointment immediately to be tested for all the STD's, all of them, and include Hep C.
Chances are, and again, I'm so sorry, these two haven't just decided at this stage in their lives, that they like having sex with other men. There is every reason to believe either one of them, your husband or his best friend, or both of them, have done this before.
Right now, you don't have to do anything.... other than see your Dr, take the time to process this new knowledge, you must feel like there's no one dependable around you.
What happens going forward is up to you, for some this would be just too much to try and work through, and that's ok, that's a decision you can make too, certainly no one would ever think badly of you should you decide this is more than you can deal with.
At the very least, for now, I'd insist that all and any contact between them ceases, and that the OM's wife is told in as compassionate a manner as is possible. Your H's reaction to total NC will tell you a lot about his perception of what he's done.
R is a very long and hard road, should YOU decide to go that way at some point in the future, with many bumps along the way. You have so much to deal with honey, take your time, breath, eat, sleep... love your little ones.
Remember at all times that this is not your fault, there is absolutely nothing you did or didn't do that could have prevented this or contributed to it. What your H has done is entirely on him, 100%.
Know that every one here is holding your hand, and thinking of you. Hugs.
[This message edited by hard_yards at 11:24 AM, February 10th (Monday)]