Cardiac stents are marvelous things. I have four of my own, and have had two of them for 11 years, and the other two for 9 years, with no complications from any of them.
Yes, you can have stents implanted after by-pass surgery. Both of my parents had by-passes as well as stents.
I'm not sure about your comment that the by-passes didn't take. I would venture to guess that they "took" but perhaps due to the scarring around them, that plaque started to build up, once again, in that area. Gives the plaque something to "grab" onto.
Wishing good luck to your mother.
There are risks in placing stents in the arteries. The arteries can tear and stents can plug up with cholesterol. Cholesterol medications can help get rid of some of the cholesterol already in the arteries. There are Lipitor studies that show this. Most likely what happened to your mother was that the bypass never did heal in after surgery. She most likely has been living with this blocked artery for 3 years. If the doctor is giving her the option of stents or medication, it most likely is not a main artery that is plugged or maybe the blockage is not severe. Ask the doctor which artery it is and how much blockage is in the artery. The heart medication today is wonderful.