I think I know why you did what you did but it wasn't in the customers interest and possibly the shops, but your interest... well more accurately your "ego's" interest. You wouldn't dare do what she wanted (nor would most people), BUT, to her it is just fine and a conflict resulted.
All she wanted was to go fishing the next day and if she was informed and ok with the cracked spool then she should have gotten what she asked for. There is no real safety issue I can see and that would be the only issue I could see.
As a result she is delayed fishing (how would you feel?) and not happy with the store... All she wanted was some line... (What happened man? LOL)
You say you took the lesser of 2 evils but in the end she dislikes the store, you made no sale, and she is delayed fishing. The other was you made a sale, she goes fishing, and if the reel brakes then she will learn herself that she may need a new reel, whether it is another cheapo or better model.
Now, if she was to "slag" your service even though she asked for it I would be surprised as she asked for it and if it was made clear customers almost always own up to it, SO LONG AS IT WAS MADE CLEAR, but if she slags anyway then..... she's a crazy customer and you get some, rare, but some. But eh, she's probably slagging your shop anyways now...
But eh, your still young... I'm 28 and I'm still learning, LOL
I'll offer you the biggest piece of advice I've learned (over years of studying it).... check ones ego (which is feeling based)... it only gets in the way of things...