Has this niece looked at the legal requirements for a Mexican Wedding? The only legal wedding that would be recognized in the US would be a Civil Ceremony, conducted by a Mexican Gov't official, in Spanish, then translated and presented to their county back in the US. There are charges for these things. Blood tests are also required.
Google is your friend here. Do some searches.
Finding a place to do a 'symbolic' wedding might be easier. Most resorts will have a wedding coordinator on staff.
As to 'where to do it', Mexico is a large and varied country. The principals here should decide what kinds of activities they want to have available to guests. Historical, water sports, golf, etc. The cost of getting the wedding party there should be a consideration too.
Good Luck with this. We researched destination weddings when DW and I tied the knot. There are hundreds of books on Amazon about it. We found that doing it in an English speaking country that has the same religious history as ours was a lot easier, so we settled on Scotland. We had both been divorced so that pretty much eliminated predominantly Catholic countries, and some wanted our previous marriage and divorce decrees translated into their own language then presented for approval before we could get hitched in their country (Greece).
A destination wedding is not something to be taken lightly, but can be immensely rewarding- well- like any wedding, I guess.