It's both things. Being an adult means not being overly bothered if you weren't invited, and it's also very adult to prevent the situation where the uninvited friend doesn't find out, out of concern for their feelings. Both are simply approaching it from different ends.
I'm in my thirties now, and I have been on my own since I was homeless at sixteen. One thing I have learned about being an adult is it means that you have no obligation to seriously consider any other person's idea of what being an adult means.
I agree. The above poster is using an example for "adult" which seems ridiculously juvenile. If I spent one minute of my life figuring out how to "smooth-over" disgruntled happy-hour-non-invitees I would be inclined to beat my head against a wall on account of the absurdity.
