I would have a no nonsense talk with a good friend about what they are going to do now. I don't think sympathetic "support" is going to be helpful to them, as it may encourage them to continue with the ridiculous situation of being emotionally "in turmoil" but continuing to divide their time between their family home and the other man.
What I'd say is, you've created a situation which is bad for you, your kids, your DH. Rather than wallowing in your upset, you need to make decisions about what to do next. Letting this situation continue is harming everybody, and it won't improve on its own.
If you want to try and stay with your family, you must cut contact with the other man, and start proving to your DH that you are committed to restoring the marriage. If you want to end your marriage, you need to have a mature conversation with your DH about doing this, and how to minimise harm to your teens. Don't assume the affair relationship will necessarily last - make a decision about your marriage in itself, not choosing between the two men.
You can't continue with the current situation, you need to make decisions. And you need to think about how to reduce the harm to your kids of whatever choices you make.
I'd not cut a good friend off completely.
People do get themselves into stupid situations. I'd hope if I made bad decisions that a good friend would talk to me in a caring but challenging way, to get me to sort myself out. But I'd not want to be full of "oh poor you" sympathy, I don't think that would help your friend in the long run.