Environmental Science allows you to explore environmental challenges through a 3 year degree (like BSc), understand the different areas and choose a path. For eg, she may start taking interest in Environmental Policy related decisions and pursue a career in Policy. Or she might decide to pursue further specialization in Green Infrastructure or Glaciology related to science. This gives more flexibility
Environmental Engineering is a 4 year degree that focuses more on the technical aspects of Environmental Conservation related to air pollution, water contamination and many other areas where you will be applying concepts in Physics, Chemistry, Biology and maths to address Environmental challenges.
Environmental Science is probably more flexible as it allows you to see challenges from not just technical perspective but also management, business and political. You can choose your area a bit later.
If she is interested in a more balanced perspective Environmental Science is good. If she wants to look at it from a technology view point engineering is better.
But there are many who have studied environmental engineering and ended up in policy.
Here are several interviews of professionals who studied various specializations including Civil engineering and ended up as Environmental Scientist.