It depends entirely on your, I mean your friend's, significant other.
I mean, if my SO did something dumb and expected flowers to fix things, it would only make things worse. Some people like flowers. Personally, I see them as the last resort of an unimaginative person, or the first resort of a person who doesn't care much.
If a gift is what you, I mean your friend, is interested in getting, get something that shows that you, I mean your friend, knows about him or her. For instance, if she's been waiting for the latest Robert Jordan novel to come out, it's coming out in January and you can pre-order it for her now, and print off the receipt for it and give it to her. That kind of thing.
Otherwise, sincere apologies are really what's important. Illustrating that you thoroughly understand what you did wrong and why it was so bad, and listening to her tell you the two or three things you didn't know about why she's upset... these things go a long way.
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I am a delicate fucking flower.
https://beacons.ai/jesskent