Alan and Henry are both in Broom Racing and Wilderness Adventures together, so they definitely know each other from at least those activities. He is soo not a fan of violence or Quidditch, though, so he's probably a little skittish of her (even if she uses her violence for good).
And Alan and Alex are in the same year, so of course they know each other! I'm thinking Alex is someone who Alan doesn't generally have to worry over, which is nice for him (except that he likes worrying over people).