I was thinking of how to help a friend improve her English, more specifically, her writing skills. 

I think three elements are the bases:
语态(voice)、时态(tense)、语感(language sense)

The first is easy, though many authors still use passive voice (because they were taught to do so in college). Most of the time, it is ok to leave it alone (when I edit a manuscript).

The second is not always easy. I wrote a Blog on this, but I still have trouble deciding which tense to use from time to time (but there is a way to go around it). The “good news” is research journals do not care about tenses as much as I do.

[Here is the link to my Blog:

http://blog.sciencenet.cn/blog-306792-657146.html

科技写作时态 (Past tense? Present tense?)]

The third is the hardest. I do not know how to help her get better, except to say: read more.

Maybe the internet has some answers? 

Here is what I found:

7 Ways To Quickly Improve Your English Language Skills

https://www.topuniversities.com/student-info/studying-abroad/7-ways-quickly-improve-your-english-language-skills

This article is about taking a test. I think the following two points are useful:

Immerse yourself in English language news (or research papers in your field)

Practice, practice, practice

At least, that was what I did; and that is what I still do daily.