If a man/woman was married in Mexico, can they get married here in the US with a US citizen?

A person who previously was married in Mexico certainly can become married in the U.S. to a U.S. Citizen. It is necessary, however, for the first marriage to become formally terminated, such as through a valid divorce, or through death of the first spouse. Appropriate documentary evidence of the termination of the first marriage, before the date of the second marriage, would be among the required papers required in an Adjustment of Status application process.

Yes, after they divorce from their spouse they married in Mexico. If it is the same person they are looking to get married to, there is no need to do so as the Mexican proof of marriage should be accepted in the US.