He wasn't going to notice her, knowingly. The truth was he hadn't noticed her until he was within a few metres of her. She was coming towards him with her friends from the opposite direction. As soon as he realised it was her, he knew it would be wise to not notice her or pretend that way in any case.
He is quite good at that now - at, knowingly, "failing to notice" somebody. He was always good at unknowingly failing to notice people - known people that is. He has got into trouble numerous times in the past for not noticing people he should have noticed, or expected to have noticed, and greeted. The fact is walking alone is such a personal act for him, he reserves such moment for himself and no one else. And even after the troubles, he hasn't been deterred from reserving this personal moment to himself and blocking out everything else in his path, except the path itself. You could say he walks in auto-pilot, with only his physical self walking the walk. Mentally and spiritually (although he says he's not entirely sure he has a spiritual self!) though, he's usually somewhere else during those moments.
On this occasion, just in a split second that he saw her face (for just enough time to realise that it was her), he saw fear in her eyes. Fear probably of him noticing her, and even worse greeting her. He realised she had noticed him all along. Probably from far off that long narrow walkway he was walking along - towards her, towards his house. That was his usual way home. Had he noticed her from afar, he would probably have switched to the walkway across the road. It was already too late, so he just had to be his usual self now - to "fail to notice" her. And, to prove her fear wrong, which he did. Or at least he thinks he did.
For him though it has never been a problem - the fear of being noticed. For he knows its just his physical self that is walking. Mentally, and probably sub-consciously too, he is usually somewhere far, where no one notices him, and no one he notices.