I hope it isn't too much to ask and off topic, but a friend of mine is learning english and I was wondering if there were any textbooks I could buy that are useful and in Japanese to learn English if that makes sense. I didn't know where else to ask so I'm sorry if this is off topic!!!

Look for TOEIC books as they are standardized testing for English proficiency. But other than that you can try searching with the word 英語 in it.