I'd think then he'd say thanks for offering...etc. Just a weird sentence construction, and given what's already slipped past him under his nose, it was a little alarming.
I've heard "Thanks for (past/ongoing/future event)" all the time. If I heard someone say "Thanks for offering to walk her home", what I'd read is that there was an offer to walk someone home and it was turned down, or had the possibility of being turned down. An offer can be rejected. The other phrasing indicates certainty.
So If Adrian made the offer and was rejected, her father would say thanks for offering to walk her. If Adrian made the offer and Miriam hadn't indicated one way or the other, he'd say thanks for making the offer (which she may or may not accept). But she'd already accepted, so there was no offer on the table any more.