Get married when and where you want, then do the spouse visa. From Mexico, it will take a couple years. If she doesn't abuse the visa, but staying too long or spending more time in the USA than out, she should be able to visit. Unfortunately, the USA does not have "I want to" visas related to immigration. You must choose from the available paths.
That what the lawyer proposes has been happening regularly for years, does not make it right. Adjusting status as the lawyer proposes, has it's drawbacks too. She'll be stuck in the USA from entry plus close to a year, without the ability to come and go, or work and will lose her B2 visa as well.