COBRA and diagnosis-related group pricing

Anyone here have any experience with determining COBRA coverage where inpatient hospital stay billed to self-insured plan under diagnosis-related group pricing (DRG)?  Participant's COBRA coverage ended 7/31 and was admitted to the hospital on the same day.  She remained in the hospital until 8/29.  Because charges were billed under DRG (as one claim dated 7/31), former employer's plan being told it is on the hook for the entire bill, even though her COBRA coverage ended 7/31 and 99% of the charges were incurred after COBRA coverage ended.  Thoughts?