How do i tell my Best friend of 15years that i and her Husband have 2kids together and he is planning of divorcing her?

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I have been besties with this girl for over15years she has been married for 10years , i have been sleeping with her husband for 9 years behind her back. Her husband and i have 2 kids together of which she knows nothing of this is because she believes i had them on one night stand with some dudes. Her husband sees his kids on a regular basis, pay support bills and my rent. My best friend is the kids god mother and her kids come over to my house for play dates and slumber party.
Now her husband has gotten the divorce papers and is waiting to file them. He wants to be with me and the kids.
My question now is how we are going to tell her... should i cause i have known her first? should he cause he is married to her? or should we just sit down and talk about it?
Please i need your help.