I am so sorry for your friend’s loss. I know how difficult it is.
Plants are always a wonderful way for remembering a loved one. We had a “Memory Garden” at my grandparents house and it was really special.
I don’t know if she kept any of her horse’s tail, but there are people who make beautiful bracelets with the horse tail.
The best gift though, just listen to your friend, give hugs and let her talk about her wonderful memories.
Bless you for caring so much.