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Correct  sentence is
     He seldom has  fatty food.
   We don't use present continuous tense to describe a person's habit or characteristic.  We use simple present tense.

Both of the following sentences are correct.  They are used in different contexts - to mean differently - as an answer to a different question.

She just had her lunch.  ---  this is in the past tense. If one narrates a story that happened in the past, then this could be used.

This is more correct - to reply to some one's question.
She has just had her lunch.  -- she finished her lunch just  a few seconds ago.
I found that he had recently gone out.
   The action of going out,  took place before  finding that.  The action of verb "go" happened before the action of verb  "find".  So to indicate that,  we use past perfect tense.

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