Your question is a very good one. I believe that even though your friend is considered in the end of her life, there are still things that can be done
to make her comfortable. Dialysis will help her kidneys, which you mentioned are not working well. It may help remove some of the excess fluid she has in her lungs, which would make her breathing easier. However, it is not a cure. It will not help repair her heart, which is the cause of her problems. Therefore, it will not extend her life, but it may make the time she has left easier.
Since I have been a nurse for over 15 years, I have seen many patients on dialysis, and it works for some people. But make no mistake, it is hard choice. For dialysis, you have to be hooked up to a dialysis machine 3 days a week for 4 hours at a time. That is more than 12 hours a week spent in a dialysis center. After treatments, people complain of fatigue.
Dialysis is a commitment, not just something you can do sometimes and not others. She will have to make the choice if this is right for her.