First of all, I think if this bag is reparable you are lucky, I don't know how they're (the repair shop) going to make it look right again. Once the velvet is gone. it's gone. If
Gucci are doing it for you than that's great and very reasonable considering the work that we know is involved. You are lucky the bag is not totally ruined.
Personally I think you did absolutely the right thing. I would have also taken the £50 because I don't think she will give you more. It's not for her to set the price, the price is the price of the repair. I also would have severed the friendship unless there's a group dynamic to this relationship. Most women know how expensive designer bags are and how, but it doesn't matter Gucci or H&M the repair bill is on her. She does not sound like someone who is loyal to you or values your friendship. From experience from lending to friends many times over and never given back (even after begging and reassurances in the face of my reluctance and talking about past experience). It was a great way of saying goodbye to the friend and to my wardrobe.
Had I been her, I would have immediately have said, whatever it costs for a repair I will pay. If I can't pay straight away I'll give to you over a couple of months. Had you said to forget about it that would be different, but it sounds like she was careless, not deliberate nor reckless but she did it. That's how most accidents happen She was the one who should have brought it up again to make sure everything was alright between you,
she should feel terrible about the incident not left you sad about the bag and twisted inside for so long. It's about valuing the friendship not just the bag, it's about valuing what you value. Considering the price of cigarettes in the UK (presuming UK since you mention £) if she can afford to smoke, she can afford the repair.