Execution plan not understanding correctly
(To answer @Michael, then segue into the OP's question.)  ORDER BY fr_user DESC, created DESC can use an index backwards if all are DESC.  Or, as in this orginal case, it can do WHERE fr_user = constant ORDER BY created DESC when you have INDEX(fr_user, created).  It simply goes to the end of the fr_user=1 entries in the index and walks backward.

But your question is "why are all 170 rows examined".  Well, they aren't.  The "170" is bogus because EXPLAIN almost always ignores LIMIT.  This is a known failing of EXPLAIN.  The Optimizer, in some situations, does take LIMIT into consideration.