I agree with the photo album idea if you have many photos of you and your friend together (or even some of your friend before she met her husband), but if you only have a few you could pick the best two and put them into frames for her husband and daughter. These, sent with a letter recalling all the good times you had together and expressing your feelings, would be a great gift for them to treasure for many years.
I have to say that my first thought was photos, but the idea of ordering a rose bush/tree on line here in the UK and having it delivered is a very good one, especially if you can include a nice message with it. Then they can enjoy seeing it bloom every year ... how lovely. Maybe do both!
Meanwhile I send you all best wishes and want to say how sorry I am that you have lost this dear friend. A similar thing happened to me a couple of years ago when one of my best and oldest friends died. She lived abroad but we kept in constant touch and I still miss her so very much. Hang in there Sue ... ❤?