He doesn't often think of witches like he is think of her now. In fact, he never really has. He's either been too busy learning something new, or.. well there were the years when nothing was learned when he wasn't even his own person. He is a late bloomer, as it were, so he feels as thrilled as any young man might feel when a pretty girl agrees to walk with them.
However, he isn't sure what to do beyond this, though he takes a cue from a passing old couple. An old wizard has the crook of his elbow offered to his wife and he leans upon him as they walk toward The Leaky Cauldron. Astrophel turns and steps backward to be standing beside her and mimics the move with his own elbow offered up.
For a moment his eyes flash dark and he almost drops his elbow, almost, almost, almost. Then, it's back to bright blue and he seems vaguely dazed before recalling she had been speaking to him.
"Oh, yes, I do rather love stars.. I excelled as astronomy in school, actually. I'm working on a rather new magical telescope that can see further than any muggle telescope can, but it's been a very difficult process. Lots of complex spellwork to be done there. Why, I had to invent three spells alone just to get where I am with it now," he isn't bragging. Just babbling as they walk aimlessly toward the likes of Memorial Alley.