Sorry to hear that.

While you're still in Mexico, your gf can still face time her mom and talk to her that way. Once she does die, all you can do is be there for you gf. Each person handles it differently. Some want to be surrounded by people they love, some want to talk through it, some want to be distracted and not think about it, some want their space and want to be left alone. You may not even know how your gf handles it if you've never seen her go through a death before. But just let her know anything she needs, she can count on you to be there for her. If she needs time off from work, you call and talk to her boss. If she's out of milk, you run to the grocery store for her. Whatever she needs, just be there.