Well, I am not sure whether this will be helpful; but I think the easiest
way how learn the essential vocabulary for a certain field of study is to buy a
standart study book for that area of study, rather than solely focused on
English for that field. I have looked it up on the internet, but unfotunately
I could not seem to find any such book. I just managed to dig up some books
that are aimed at those students who are studying a course within the field of
enviromental science (I cannot send the link to the discussion up here, so
I have send it to you into the message box on the homepage). Books designed for
CAE students, such as English Vocabulary In Use, includes the technical
terminology from many fields, for instance, economics or enviromental science.
I don't know to what extent your friend needs to know the vocabulary of this
field, but in my eyes this sort of books are very English-students-tailored
rather than being a comprehensive overview of a field's terminology. Talking
from experience, I don't think they are optimal – I found economical
terminology in the CAE book, which I need to know since I study it, but it
includes just few umbrella terms. So, as I've already mentioned, the easiest way
might be to buy some of those book I've mentioned, whether a book focused on the
terminology of the field or just a standart study book, which often includes a
terminology, often supported by appropriate diagrams.