Some women just seem to bleed heavier than others when it comes to their periods, and it does seem to be a hereditary thing. If the Mirena isn't working after that length of time, then it's probably fair enough to say she needs to look at other options. Women bleed more when they start going through the menopause, and their periods get closer together. Just make sure the doctor has checked her out for fibroids and endometriosis, as they can also worsen bleeding. If she doesn't want to go the whole way of having a hysterectomy she can have an endometrial ablation, where they destroy the lining of the womb to reduce the bleeding. This is not 100% effect either though. The only thing that is is complete removal of the uterus. There are also side effects of anaesthics, so her doctor will need to assess her health in regards to that also.
I am trying Mirena as well at the moment for the same thing, incredibly heavy bleeding, but I'm only 3 months into it. The bleeding is reducing for me, but I am having some side effects that I'm concerned about. If this doesn't work for me, then I will be having a hysterectomy. Life is too short to live it in pain and the discomfort of heavy bleeding all the time. I wish your friend good luck in her decision.