Can someone give me guidance on this issue?

Friends of mine purchased ET codeshare for honeymoon in Y from EWR to JNB with initial leg on UA metal EWR to IAD. Due to weather, they were rebooked to JFK to JNB nonstop on SA metal. Baggage couldn't be retrieved at EWR though. Who's at fault and what is the compensation policy for delayed baggage? I assume UA because they're originating flight. They're is no guidance on UA site about incidentals and reimbursement. Does anyone have details on this?