I experienced so similar a situation three years ago as your friend is experiencing now. The difference between the two men was that your friend's hu*****and told her his feelings and plans in the letter, my hu*****and said and practice the similar things gradually.
I am in my early forties with three young kids. When I was told his affair with the other woman (a married woman, his colleague at that time) by that woman’s hu*****and, my youngest was only 5 months old. I was in a complete shock. I tried to persuade him not to divorce me, but in vain. So I hired a lawyer to draft a divorce letter and insist that I will keep all my three children. Since my income is similar to him, the lawyer suggests that we should equally distribute all assets we have and he will pay the child support until all kids grown up. The child support suggested by my lawyer was around $2500 a month at that time. I still remembered so vividly his response: “NA4 ME DUO1 so much?! NA4 WO3 HE2 TA1 DE1 HAI2 ZI1 ZEN3 ME BAN4? How about my future kids with her?” However, several days after that, he somehow come back home, and decided not to divorce. He said that he loves hour kids, wanted to be responsible for them. I welcomed him home at that time, but I now think that might be a very big mistake. Since he think that his affair was my fault, and he sacrificed his happiness to be home with me and the kids. He demanded complete obedience from me (which I tried, but not very successful). Because of the job, I have to relocate to another state two years ago (I always wanted to be a stay at home mom before his affair with that women, however, he think the family needs two income. And I dare not to be jobless since his affair with the other women. The relocation was unavoidable because of the job). I brought my kids with me, and he drove 4 hours every week or the other week to visit us during the week ends. He refuse to relocate to join us, although, he had the chance. And now, I just found he is still keeping very close contact with her, I do not know exactly how close they are, since they were using Skype or yahoo message to communicate. He knew that I can check the phone bill of the cell phone so I thought he would avoid using the cell phone to contact her. I did not pay attention to our cell phone bill for a long time. However, I do not why, I somehow decided to check the phone bill last week, and found that he called her several times this month. So I checked the historical bills, and found that he was calling her 8 times or more every month (Since we switched cell phone carrier, I only have the online statement started from Feb 2010), all during after work hours, and every call lasted only 1 minute. I am in such a great pain, I really wanted a divorce, and however, my friend is trying to persuade me not to. She said, a broken umbrella is better then no umbrella, a bad hu*****and is better than no hu*****and. She is a wise woman, I knew she was right from the practical point of view and it is the best for the kids, however, in the reality, the life was so painful for me to stay in this marriage. I still could not make up my mind about what to do. Hope your friend is stronger than me.