Some people with anxiety have a hard time seeing people outside the context they're used to seeing them and they might have a panic attack if they think they have to talk to someone.
Now, I don't know if this is your friend or not
I'm not even really excusing it ,sometimes we have to rise above our fears and our own personal problems.
But if he has anxiety the above is a possible reason why.

Here are a couple of examples I know of . Neither one might be the explanation in your case, but they give an indication of the kind of thing that might be going on.

A friend of mine was walking down the street and saw a woman he knew coming toward him. He waved and smiled, but she kept a stone cold face, avoided his gaze, as she walked into a liquor store.

She has a drinking problem. She was ashamed of what she was doing and so did not acknowledge him. Or , perhaps another way to look at it, she was leading two lives (as addicts tend to), and a "crossover" between her two lives was stressful.

As for an example involving myself, I remember when I was a student and knew another student through the Catholic organization on campus. She eventually stopped going to Mass, and when I would see her in other settings she would act as if she did not see me.

We all have multiple aspects of ourselves within us. Sometimes we freeze up when two of our worlds collide with each other.

My dad used to place great store in "<whoever> always knows you when you see them [out somewhere]"...and I think your negative experience shows why. 🥺

If the person has bifurcated sets of friends--not wanting the sets to collide by knowing of each other--then it makes one wonder why: is one set not "good enough" for the other? Is it a form of "double life"?

You might ask, but don't expect the truth. Sorry. 🥺

It could mean many things. For example that they don't want to deal with anyone at the moment or they are with someone or wait for someone and don't want them to see them with you. Idk, people can be pretty weird and asocial. I know I am but still..I don't think I would do this to a real friend that I like. As you say, this person is just "friend".

I did this once. But I felt guilty about it.

I noticed someone I knew on the platform at a London railway station and pretended not to have seen him.
I could not bear the thought of having to sit with him on a long train journey as he is so boring !

In retrospect, I should have acknowledged him briefly and then pretended that I was travelling with some friends in another carriage.

People have their moments sometimes. Maybe they didn't want to be bothered. Who knows. Being i am who i am i would have reached out said i know you saw me are you good? I would put the ball in their court and just leave it there. Its up to them whether they decide to pick it up.

FYI, those aren't friends. Perhaps they used to be, but now they aren't. I would suspect that one of those 'so called friends' is spreading rumors about you out of jealousy. If you feel closer to someone in the group, ask them what's going on.

Don't worry. I have a collegue now that pretends not to see me. He's done that already three times in public. Either he turns away or stares out. He doesn't like me. The problem that he has is other people don't like him either. It all comes back at the person

It means we aren’t friends and never were. Not sure if it’s them ditching me or me mistaking mere friendly behavior for friendship, but whatever the case, I can take a hint. Nice knowing them. 🤷‍♀

Could mean that they didn’t see you.