Wecome to the forum! Is your friend entering the US with a visa, or under the 'visa free' program. If it is the 'visa free' program, I think Mexico counts as leaving the country, and it affects whether you can get back in again. I think this solely because someone told me he had a problem doing that. It's something your friend should check out, anyway.

You should also verify whether a 90-day visa keeps running when you are outside the country. In general, multiple entry visas are valid 90 days from the day you activate the visa, so leaving the country doesn't extend the visa, even if it is counted as a border hop. So if a visa is originally set to expire on the 5th of February, that's when it expires, even if you left and spent a month in another country. So that's another thing you need to verify.

Have you looked at the US State Department site, U.S. Department of State - Home Page