Ministry: Department for the Regulation and Control of Magical Creatures
Amos Diggory is Cedric's father. He's a friend of Arthur's at the Ministry, and they seem like they might be on roughly the same professional level at the office. When Mad-Eye Moody almost gets arrested by Muggle policemen for his exploding trashcans just before he goes to Hogwarts (which is actually the night when Barty Crouch, Jr. takes Moody's place), Amos Diggory thinks to call on Arthur Weasley to fix things with the Muggles. So there's definite respect between the Weasleys and the Diggorys.
Amos Diggory is hugely proud of Cedric (as any father would be). This explains why he's sometimes so rude to Harry. From the start, when the Diggorys and the Weasleys catch the same Portkey to the Quidditch World Cup, Amos Diggory needles Harry about having lost to Cedric in Quidditch last year. It only gets worse once Harry and Cedric are competing for the Triwizard Cup. Amos appears to be much more competitive with Harry than his son is, in fact. When Amos Diggory comes to Hogwarts to watch the third task, Cedric explains outright, "Ignore him [...] He's been angry ever since Rita Skeeter's article about the Triwizard Tournament – you know, when she made out you were the only Hogwarts champion" (31.92).
(Click the infographic to download.)
Once Cedric dies, all of that resentment passes away in the face of Amos's terrible grief. When Harry speaks to the Diggorys after the third task, "Mr. Diggory [sobs] through most of the interview" (37.2). He doesn't blame Harry at all for Cedric's fate. He just seems too overwhelmed to talk about his son yet, which is certainly understandable. Harry's meeting with the Diggorys is one of the saddest parts of the whole book, particularly since their son died during the Triwizard Tournament – the competition that was supposed to be one of the proudest moments of the Diggorys' lives.
We shudder a bit with horror when we think about what it must have been like for the Diggorys sitting in the stands and watching as Harry Potter suddenly appears, holding the Triwizard Cup and the body of their young son. It's got to be a parent's worst nightmare.