exit visa and medical test requirements

My friend is living in Nigeria on a student visa and she wants to leave Nigeria and get to her country USA. She's diagnosed and she was told by her university staff that she should get treated before leaving and she can't leave if she doesn't get treated. I don't understand why must she get treated before leaving Nigeria. She can get treatment in her country whenever she wishes. Can you please explain to me about it? Is it true?

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