You've made the right choice. You simply must tell her. I do not envy you. This is going to be awful. But you are 100% doing the right thing, and she really needs you right now. Do it quickly now; get it over with as soon as possible.
Be as honest and compassionate and empathetic as you possibly can, but tell her. Put yourself in her shoes and ask yourself how you would want to hear that news - where you'd want to be, what words you'd want to hear.
Do it in person. I assume you live near enough to each other that you can pop round to see her? I wouldn't tell her over the phone; I'd do it face to face if at all possible. Call/text and ask if she can meet you somewhere or if you can come get her and take her somewhere. Be insistent. You don't want to freak her out too much before you tell her, and you don't want to have to tell the poor girl over the phone if you can avoid it at all.
Go to a quiet, private place where you can be as alone as reasonably possible - no DC, no DP, no other friends - so that she doesn't have to cry in front of others. Go somewhere where she won't have to feel self-conscious, if you can find it. I don't recommend her home, for sure, and probably not your house, unless your DC and DC are out and you can be alone.
Things to take with you: Bring tissues and baby wipes in case her make up runs, and maybe even take an extra top or cardi for her in case tears stain her clothes or something. Maybe take a couple of drinks with you? Something light and fruity and tasty, or some wine (those little single-glass bottles or something might work). You don't want to get her hammered, obviously, as she'll have a nasty confrontation on her hands when you part ways, but something to take the edge off hidden in your purse would be a god-send in that moment. Bring her favourite snack or chocolate or something like that, too.
Be prepared. Get all the proof you can, so that you can try your best to assure her you're not telling her a lie to hurt her or anything like that. I know that sounds awful, but hearing this kind of news can make people feel desperate and panicky and paranoid and irrational at the best of times. If you don't have any proof apart from what you've heard, then tell her "listen love, I heard something horrible and I need to tell you. I don't have any real proof beyond this, but my DH told me that your DH told him...."
Be ready to support her, hug her, slag him off with her, and even be ready for her to be angry with you. That will pass quickly, I'm sure, but in the moment her feelings will be in control, and at the top will be pain and anger, and you'll be her only possible target. Just be as compassionate as you can - try not to get defensive - and let her let it out.
She'll probably ask when you found out. You've got two options there:
A) Lie. You tell her that you only found out yesterday and came to her as soon as you heard. You'll need to arrange this option with your DH as she may ask him for corroboration, and if she finds out you lied she may be hurt.
Or B) Tell her the truth. Tell her that you wrestled with telling her because you didn't want to hurt her, worried that it's possible she may not even want to know, worried that she already knew and didn't want it to get out, but you realised that you simply could not let it go without telling her because she is your friend and you love her and she deserves to know. She may be hurt by the fact that you didn't tell her straight away, but in the end she will be grateful that you told her.
I think, personally, that honesty is best here. She has been deceived more than enough already, the poor girl.
Ohhh good luck @Mumstheword. This is going to be awful. I'm so sorry you've got to be the one to do it. Be strong, for her. She really needs you now. You're a wonderful person for doing this.