What is the ruling on translating pages from an accounting book from a foreign language, being careful not to translate any forbidden words? And what should have been done?

What you did by helping your friend translate what he wanted is a good deed for which you will be rewarded, God willing. The basic principle is that translation is permissible, unless the book contains something that is impermissible to translate. If it does contain something impermissible to translate, and you avoided translating it, then you have done your part, and there is no sin upon you. You should also warn your friend against studying what is impermissible, and explain to him the reason for not translating it.