So sorry for your loss, Jen. To lose a cherished friend is a terrible blow. I understand. My dearest friend died two years ago, and not a day goes by that I don't mourn her loss. But also, not a day goes by that I don't thank whatever powers may be for letting me have such a friend for so long. And that's the part I try to focus on. Mourn her, because people deserve to be mourned. And then remember her for all the wonderful things that made her uniquely her.
Let me pass on an idea for a gift to her children after they grow up. When my friend died, I started jotting down little memories of her. Just everything I could think of that she always would do or say, trips we made, things she liked. After a year, it was about 30 pages. I wrote it up so it would be intelligible to someone else, then gave it to her adult daughters. Of course, I asked them first if they would like it. Perhaps you could create such a thing now, to give to her children when they get older?
And re your knee, three weeks was the same time I started to feel vastly improved. I heeded all the warnings not to do too much too fast, but that was when I realized this probably was going to work! It's a great feeling.