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I am sorry to hear about this situation.
In Michigan, the landlord cannot enter the apartment without proper notice unless there is an emergency. See HERE. This includes any agent or contractor acting on their behalf.
What the manager did was improper. And bringing the neighbor was also improper unless the neighbor was acting as the contractor for the supposed smell. But even if they were, lack of notice is still improper.
If she continues to do this, then:
1) You/your boyfriend can get a restraining order against them - see HERE; and/or
2) The boyfriend can claim constructive eviction and quit the lease without penalty, and then seek the deposit back via small claims.
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