So guys, I was just reading the character profiles for ITF and look what I found:
"Rather than be open with his Dad about his close friendship with outsider Kieren, Rick joined up. The army was perfect for Rick; it was an escape from his feelings and his father’s judgements, that is, until he was killed by an IED.”
I don’t know if it was just me that didn’t realise but Rick joined the army so he didn’t have to tell his dad about him and Kieren.
So that’s why Kieren felt guilty for Rick’s death…because it really was because of him that Rick joined the army