I know someone who just married a man to obtain a green card and then about 8 month later got pregnant by another man?what is that?is it legal?

That is called infidelity. It's not moral, but I doubt that it is a criminal offense.

What does the man in this situation say? If he is not considering going to Immigration about the situation, what are you planning on doing and why is it your problem?

I wouldn't call getting pregnant to another guy immigrant fraud. The only fraud would be if she married him ONLY to get the green card but then as long as she is willing to stay married with him it still could not be proven as fraud. If she is cheating on him and there is documented proof she is cheating on him then maybe there could be a case of fraud.
Her just getting pregnant to another guy I don't think would be enough to prove fraud.

i know someone who just married a man to obtain a green card and then about 8 month later got pregnant by another man?what is that?is it legal?

It's call infidelity not immigration fraud.

However, if her husband files divorce on ground of adultery, and marriage is terminated before she could file a remove condition, then she will be out of status.

Will the baby is related to me in a way.the man she married has been in jail sinc before they got married.not only dose she have this problem but recently she got a job as a realty broker/notary public and she has been doing or committing fruad through the commpany.

What does the guy she married say about her being pregnant and it not being his?
Proving she is committing fraud in her job may be a problem if it is proven but her relationships really would not go anywhere as fraud as it stands,

will the baby is related to me in a way.the man she married has been in jail sinc befor thay got married.not only dose she have this problem but recently she got a job as a realty broker/notary public and she has been doing or commiting fruad thru the commpany.

How long has her husband be in jail? If he is expected to spend a year or longer in prison, then she has strong supported reason to file a divorce and remove her green card condition without the US citizen spouse.

How sure are you that the kid doesn't belong to that incarcerated guy? I believe married inmates are given conjugal visit.

The immigration officers were experienced enough to know what marriages are done in faith, regardless if she conceived the baby through other man, so leave their job. Same as with the company she works at because there are auditors.

Many businesses get embezzled by thousands of dollars months and even years before an auditor catches it.
Only a small percentage of jails have conjugal visit I think it is something like 5 or 6 states and then I think that is prisons and probably not jails. Excon can probably clarify the details on that.

If of course they married for fraud as long as you can prove that. But to be honest if the person who married them does not care, why should anyone

If the immigration proved that the marriage is "fake", the woman can wave bye-bye to that green card forever because she will never get one, even if another man will petition for her within the US and she will not be permitted to enter the US again, ever.

Like Fr_chuck said as long as the husband is okay with the marriage then it would take a lot to prove it is a fake and who would really be interested in going to such great lengths to prove it?
I think it would be easier to prove she is stealing from the real estate company.

You weren't so clear about your question that's why we have these many angles.

I agree with Nohelp4u.

By the way, just to add. If the inmate aware that their marriage is fake at the very start, he'll pay a lot of money and maybe extend time in jail.