Your friend is wrong.
Yes, there are areas that I recommend being more cautious and aware of your surroundings but that goes for a lot of places worldwide.
While I went ahead and just got the easy-to-carry passport card, if you have the regular passport book, score a cool cover for it.
You may get change back in pesos, though! Credit cards are accepted at bigger restaurants and the nicer hotels but TJ in general is a cash economy.