My prayers are with your dad. I know this can be a scary thing, but let me offer some comfort. My dad had 5 bypasses done about 10 years ago. He was never one to go to the doctor, went to work one day and they convinced him to go to the hospital. When he went, the doctor immediately took him in for bypass surgery and said that if he didn`t come in, he would have never made it through the night.
The surgery went well...the only thing that was a minor hiccup was that after surgery, one of the bypasses that they did, had a minor leak, so they went in and fixed it. To date, and at 75 years old, other than having developed diabetes (which I understand is a side effect of heart surgery), things are going great for him. Some other minor issues, like his hips and joints are starting to ail him, but other than that, no major complaints.
The doctor told us that this is such a common procedure anymore, that when they schedule them (not emergency like my dad), he (my dad`s surgeon) does between 7-10 per day, several times per week...so as scary as it sounds, it`s a common procedure.
My wife was diagnosed with breast cancer and had surgery done. Obviously, it was big for us and scary...her surgeon told us that he does around a thousand of these procedures a year, which was somewhat comforting to us and a year later she is doing well.
Although, when I broke my back several years ago, my doctor told me that there are no guarantees in medicine (and if I wanted a guarantee, to go to Sam`s Club), but he did promise that he would do his best. I am sure that holds true for most reputable doctors.
I guess what I am saying is that prayers always help, but if you are comfortable with your doctor, have faith in him; as this is not an uncommon surgery, and I`m hoping that everything works out well for your dad!