I may have caused my former friends decent into alcoholism
What makes you think that it is your fault?
I've never talked to them after the gap year and the gap year is when they started drinking
That may only be a coincidence!  Regardless, you shouldn't blame yourself for choices that another person freely made.  Ultimately their choices are their responsibility!
What if I agree with you, but still feel like shit because of their situation.
Try not to beat yourself up too much. I know it is hard, but you should not blame yourself for others problems. You cannot control what they do.