My dad had a quadruple bypass almost ten years ago at age 66. He had zero complications, just the general pain of recovery as he healed. Granted he's younger than your mom, but the procedure is very common these days. My uncle had a massive heart attack and died in an airport. So I wish he'd had the opportunity for open heart surgery. Surgery is scary, but probably best way to go. Even if the risk is not an acute event like a heart attack the benefit probably outweighs the risk. My grandfather had some sort of artery operated on in his 80's when I was a kid. I remember the difference before and after he was like a new person when normal blood flow resumed to his brain. Before that I could barely have a conversation with him. After he lived to be 96 arguing politics and getting along fine.
I had a mitral valve repair (open heart surgery) and a maze procedure done at age 67. I don't recall any doctors telling me there was a high risk. Of course there is a risk, but as I was diagnosed to die of congestive heart failure in about 5 years if I didn't have the surgery the risk/reward said get it done. It is now over 5 years since surgery and my heart is still pumping, albeit with permanent atrial fibrillation that is well controlled..
I have no number or even any idea of how many who were in for heart surgery were in intensive care... I was myself in intensive care but that was part of the progrem. I had no problems but was connected to a few tubes and always to a heart monitor for 5 days before released. I had next to no pain and was sitting up in a high back recliner within a couple of hours after surgery. Yes, I was in good physical condition before surgery, always have had good resistance to whatever virus is going aroiund, and was "only" 67, I was still a runner for exercise up to about 4 months before my surgery.... said another way I'd expect someone in their 80 will find the surgery much more taxing than I did in my 60s. A neighbor's father had similar surgery to mine when he was in his early 90s. He had a much longer recovery than I did, but he has recovered and is still with us at about 95.
We have to make our decisions on relative risk and benefit, there are no guarantees.
I have had open heart surgery and know many people who have also had the same thing, and one thing I have learned is one of the biggest risks, infection. When the body goes through such traumatic surgery, it becomes weak if there are any infections in the body, even if there is a tiny amount. When in hospital one day, I counted 10 people go in for bypass, and only 7 returned later in the day, the other 3 were in intensive care on life support. There have been many discussions in the last few years among surgeons if patients should undergo a course of antibiotics before surgery, to ensure they are free of infections, but of course antibiotics don't work with everything. One of the best methods to detect if your Mother is currently fighting any infections is to have a blood test, a full blood count to see if her white cells are high. This would almost certainly mean that her body is fighting something off, and I would delay until the infection is gone. If the infection spreads to the lungs after surgery, as most do, then pneumonia is developed and becomes life threatening. However, with regards to the surgery itself, surgeons always say it is risky, they have to. It is risky but they have the best equipment around them to deal with any unforeseen problems. If your surgeon has performed such procedures many times, hundreds of times, then this experience will considerably lower the risk.