And Seamus doesn't even come down against Harry. He just brings up the fact that his mother believes the stories in the paper and Harry immediately makes it about Seamus choosing Harry over his mother when it's not really necessary. I think Seamus was open to discussion and was curious to hear what Harry had to say before Harry (understandably) snapped at him. Then Seamus was shown as wanting to reconcile later, but Harry only accepted it when he showed up as a member of the DA, not as a friend, but an army member.