How can you wait after completing PADI classroom til you can take your open water dives and be certified?

My family is going to take PADI diving lessons in a week and my girlfriend is starting to get really scared of the ocean because she has never been in it. She wants to take the classroom/pool dives but is not comfortable yet with doing the open dives in the quarry. How long after the classroom/pool can she wait to take her open dives and still get certified? We are going to Mexico in January to go dive, would she be able to do her dives then?