“Not a right fit” is oftentimes agent-speak for “not good enough.” They don’t want to crush your feelings by saying your stuff isn’t ready for prime time so they use “Not the right fit” as an easy way to let you down.
Now, sometimes an author will send them stuff that they don’t represent…for instance fiction to a non-fiction agent. So in that case “Not the right fit” is EXACTLY that…and the author is at fault because they didn’t do their research.
To be honest, most agents don’t get far enough into the piece to know whether the plot was strong or not. Most agents decide before the bottom of page 1 that it’s not good enough. That sounds like a ridiculously short period of time. But trust me it’s all that is required for 95% of the pieces. For years, I’ve offered a free “first-five pages” look for new authors. I’ve looked at more than 500 openings, and I think I found 1 (maybe 2) that I read beyond page 1. Ofttimes, I can tell in the first paragraph that more practice is needed. There are some times when I can tell in the first sentence. Have you ever listened to someone trying to play a musical instrument who aren’t yet skilled at it? Your ear picks up on it immediately. The timing is off, the notes are missed. It’s exactly the same way with writing.
If you’ve not heard this yet, you will…assuming you stay in the "publishing game long enough. “Ideas are a dime a dozen, it’s execution that counts.”
Agents often take writers under their wing…but it’s not to get their writing skills up to speed. That is something only time and practice can help, and until you have that mastered, they can’t help you…after all you may NEVER get to where you need to be.
Where they provide assistance is with fixes to the plot (or characters) that can make the book stronger. They might say…it lags in the middle, cut this scene or that one. Or they might suggest other “high-level” changes.
As I said, agents “take chances” all the time. The “risk” comes from the fact that most books fail to earn out (Only 20% do). But I’ll venture to say that your friend wrote well and had a good story to tell. If that wasn’t the case, she wouldn’t have been publihsed