I had my first hip replacement at 36 and second at 38 due to juvenile rheumatoid arthritis, and it has made such a difference to my life. At the time of the first, I was a mother of a 2 year old and a 4 year old and couldn't chase them or really play with them at all due to the pain. Just to be off all pain medication is fantastic and I feel so much healthier for it. I am scared to go horse riding or ice skating (I probably could go) but it is a small price to pay to be pain free after so many years.
Tell her to ask lots of questions about what she can and can't do afterwards, but I would go ahead as she hasn't got anything to lose with the knee causing her so much pain anyway.