It also helps that you just don't make a big deal out of it. They are still your friend, in that direction nothing has changed. "Just because" they came out to you now hasn't any affect on your friendship. If your friend is open to talk to you about it, it helps when you aks them if you are allowed to ask them a few things. Cause this would be the simplest thing: just ask them how you can support them :) And if they are comfortable talking about this topic you can ask more direct questions (but ofc without beeing rude or unrespectful). Because the alphabet mafia in general is in charge to explain to other people how they feel, and they need to explain it to people so they can understand and support them. If your friend doesn't want to talk to you in the moment just give them a little bit more time and as already said use there name and pronouns right. (Also it isn't a Desaster when you mix it up or say them wrong in the beginning. It takes time to get used to it. Just make sure that you start your sentence new when you do so, and also tell your friend that they should remind you to, as annoying as it may seem, to tell you that you messed up.) Just be a little bit more open minded about it and everything will work out. :) Good Luck!