UM/UIM and the Other Insurance Clause

Insured #1 was driving her boyfriend's car.  The boyfriend's car insurance is with Hanover and provides $50,000 liability, um, uim. 
Insured #1 also has the $50,000 coverage and it is with Columbia Insurance.
The at-fault, deceased party is also insured with Columbia with the same limits as insured #1 and her boyfriend.
None of the policies provide medical payments coverage.
The deceased crossed the center line and hit insured #1 head on. He was killed in the accident. Insured #1 was seriously injured and incurred several thousands of dollars in med bills, missed work for 3 months, and has some permanent injuries.
Columbia paid the policy limit of $50,000 for liability of the deceased driver but contends that the underinsured motorist coverage it has under insured #1 policy is excess of Hanover 's policy. 

Hanover is taking the position that its underinsured coverage on the boyfriend's coverage is excess of Columbia 's.
Whose underinsured motorists coverage is primary?