That's great that you get to borrow your friend's Winston Grammar!
We struggled with grammar for a long time (trying almost everything--but not Winston, LOL), and then we used Easy Grammar for a year (which really helped!), before switching to ACE (Accelerated Christian Education) English several years ago. That has been the only program that really "clicked" for us, and dd has grown tremendously in her grammar skills. Lots of repetition (which helped), and gets incrementally deeper and deeper until they are doing high-level grammar quite easily (for the most part!)
Love in Christ, Heather (WI) ~enjoying FIAR products since 1999~