My friend's kidneys are affected 85% due to an artery blockage. Is there an operation to fix this?

Yes. The most common way to address blockages in the arteries to the kidneys is with stents. The procedure is straight forward and done as an outpatient. The harder question is, "does he need it fixed? " if his blood pressure is really difficult to control or his kidney function has been worsening, than he would most likely benefit from treatment. Otherwise, the need for treatment is questionable.
Angioplasty. This is a minimally invasive procedure performed by interventional radiologists. Blackage is first expanded with a small balloon which is followed by stent placement which keeps the blacked area unblocked. Renal artery bypass surgery, standard of care before angioplasties were performed, now virtually never done. Subjects mostly need to remain on antihypertensive medication even after angioplasty.
Renal stents. Ask your doctor if renal artery stents are an option. Nephrologist will know.