Charlie has promised to talk with some old friends on Katie's behalf. She's got at least one contact of her own in Oliver Wood, now quite the rising star, but it can't hurt to have a little extra help.
So he's going to have a word with a few people who were on the Gryffindor Quidditch team with him. Extol her virtues as a Chaser; point out her unbroken winning streak lasting several years (although he won't tell them who was Seeker on the team during that time). That kind of thing.
He won't mention his vested interests, of course.