You say Angioplasty AND bypass but I'm hoping you mean OR?
Have you been given both as options? What is his quality of life like? how much discomfort is he in?
I think asking such a question is bound to get different responses because we all seem to have our own preferences and opinions on intervention treatment.
If his condition is controllable using medication and knowing he has collateral development, maybe lifestyle changes will be adequate.
If medication is not severe enough in intervention, then my personal opinion is angioplasty over bypass if available. I only say this because of my own experiences. I went through a lot of discomfort for the whole procedure to give me benefits for just 3 months. The Stent I had in my LCx is still fully patent after 3 years. However, I must emphasise that you should try and obtain a cardiologist with a lot of experience in dealing with large blockages. I would also ask them if they can remove as much of the plaque as possible before ballooning a stent into place.
As I said, this is a personal opinion based on personal experiences.