I'm going to call out my friend for making zero effort to introduce his girlfriend of 9 months to any of his friends
That's quiet a stretch, how did you know that your friend made zero effort?
I'm his friend of 15 years, he introduced me to his family and I've never seen his girlfriend before.
You definitely should be suspicious of your friend if he can't show off his girlfriend to everyone. Maybe he is just faking her existence. You need to get the truth out of him and call out his lies.
Nah, he will make excuses and I'll take the blame for harassing him.
I thought you wanted to call him out? Even if you have to take the blame, I believe it's good to try to bring some sense to him and his actions. He can't keep deceiving himself and others, and maybe you talking to him can make him stop and find someone for real.