Is this a Special Enrollment Period?

Wife is 65 and has Medicare A & B PLUS employer coverage for herself and her 63 1/2 yr old dependent husband. She wants to enroll into a Medicare PPO with me for herself and get an ACA policy for husband. Does her VOLUNTARY dropping of the coverage create a Special Election Period for hubby to enroll into an ACA policy? Her Part A has been effective since 2011, but her Part B only started 12-1-15 or 1-1-16 (I need to confirm). I'm concerned that his SEP ONLY applies in situations where there is a job loss or INVOLUNTARY loss of insurance.

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