For all you fitness people out there, can you tell me if this diet is safe?

So my friend recently told me about this diet and it goes like this; 2 days of 1300 calories and 1 day of 900 calories. Vigorous exercise on on the 2 1300 calorie days and none/minor exercising on the 900 calorie day. You repeat the process for at long as you want. Does this diet sound safe?
For all you fitness people out there, can you tell me if this diet is safe?
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