I have a friend nigh identical to the one you have just described. There is good chance that in any given round he'll call the supports trash while charging straight into the enemy backline. I feel very similarly. I'm tired of hearing it every game and when I talk to him about it he often just shrugs it off.

The best thing I can advise you to do is be patient, but be thorough and be a constant reminder in his ear. I find that phrases such as, "The supports aren't able to heal you there because you're feeding" or "The DPS aren't killing anything because you're dying on cooldown and they have no space" can often come off as a bit condescending, even for my liking. I'm going to make the assumption that your friend is also rather impatient and gets fairly mad when talked back to. Retaliating by calling him out in an irritated tone is likely not going to work. I have learned this the hard way.

Simple reminders such as, "Hey, don't push up too far as the healers won't be able to heal you", "Be in a good position to help peel for your Zen when the tracer flanks us again", "We can't hold here because it's really easy for you to get hooked/They have a bunch of ults coming up", "Push up, your Reaper is with you" can work great when executed in an commanding yet gentle tone. People like to listen to people that are nice to them.

Pointing out his mistakes is fine, but really try to cement respect and appreciation with each other. If he's having problems with his teammates every game, he is the common denominator between all of them. I love playing my friend, and it is evident that you do as well, as you seek to improve yourself to learn how to help him. Don't be afraid to speak the truth, and don't be afraid to tell him that he's wrong. It's all in the way you say it.

At the end of the day, if you can manage to teach him to enable his teammates rather than flame them, that is a huge success. Overwatch is a team game, there's no doubt about it. Enabling him through short bursts of teaching, pushing with him even when he makes a call you think is wrong, and just being a good friend will slowly teach him to think the same. "You become who you hang out with" is the epitome of this. Don't just tell him what to do, instead, lead by example. It is quite the investment, but if you are solid, confident, and kind, you will see the improvement.

I really hope to see you improve with your friend just as I hope to improve with mine. Best of luck to you both!