You gave them your advice and some reading material. They made their decisions based on their own attitudes and comfort level, which is perfectly acceptable, and well within their rights as parents. I don't always agree with the views or choices my friends make, but their children are their business, and what will ultimately make them good or bad parents involves way more than their birth choices and nutritional decisions in the first months.
I get that it's hard to see other people have (often without appreciating) what you would have liked the opportunity to have for yourself. I went through absolute hell to try and nurse my kid, and there was never a time I didn't need to supplement, despite all my hard work. I cried when I pulled out a bottle in public, which in retrospect, is really, really sad. Do I wish I could EBF? YES. do I envy others who have the opportunity? YES. Do I resent mothers who choose a different path, for whatever reason that might be? NO. It's just none of your business, and they have no obligation to you, or anyone, to do things differently than how they WANT to. The fact that you're "heartbroken" for their babies does indicate that you're internalizing their situation, when there's no need. How do you think your friends would react if you told them that you're "heartbroken" for their children? How would that make you feel, if it came from one of your friends?
I can guarantee you that you will always differ from other people in your parenting choices. Some people will look upon your choices as imperfect, or flawed. It's not their business, though, so long as you're doing your best, and only YOU know what your best is. Same goes for your friends. Parents should not be made to feel badly about their birth choices, or their early parenting choices, so long as the baby is safe and healthy. Parenting is a very personal thing - there is no universal "right". Ever.
You can't help how you feel with regards to what you've missed out on. I totally get that. I just think that showing up at a hospital to congratulate your friend, and saying something that warranted a brand-new father having to JUSTIFY their choices to you, indicates that you need to step back and think about what you can REALLY do for them as a friend at this point. Usually this involves bringing over food and cooing over how cute their baby is, and leaving them be regarding the parental evaluations.
It is harsh. I just feel there's a big difference between struggling with envy and longing, and judging your friends for things that are really, really none of your business. I hope you can find some enlightenment to this end, and continue to enjoy your friendships.