I suggest you go to www.dmwv.org which is the Descendants of Mexican War Veterans website. This organization will not do any work for you but you might find some links or ideas of places to look on their website.

If your person is a Texas soldier, I recommend TEXAS VIOLUNTEERS IN THE MEXICAN WAR by Charles D. Spurlin (Eakin Press: Austin, Texas) 1998. because it has a printed list of Texas soldiers taken from the rosters held by the Texas Archives and National Archives.