A B757-200 ACFT PAX NOTICED R INBOARD LEADING EDGE SLAT; CENTER TRAILING EDGE SKIN AND HONEYCOMB SECTION SEPARATING FROM THE SLAT IN FLT. EMER DECLARED.

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APPROX 300 MILES W OF ZZZ AT FL350 WE WERE INFORMED BY THE PURSER THAT A PAX WAS RPTING PART OF THE ALUMINUM ON TOP OF THE WING WAS PEELING BACK. WE HAD A DEADHEADING 757 CAPT IN THE CABIN AND HE WAS INSTRUCTED TO VIEW THE WING AND RPT BACK. HE CAME INTO THE COCKPIT AND RPTED A FAIRLY LARGE SECTION OF THE FAIRING COVERING THE LEADING EDGE SLAT BETWEEN THE FUSELAGE AND R ENGINE WAS DISINTEGRATING. I AM AN LCA AND WAS PERFORMING A 24 MONTH RANDOM LINE CHECK ON THE CAPT. I WENT BACK TO OBSERVE THE WING AND FOUND A 12 IN X 8 IN X 2 IN DEEP SECTION ABOVE THE INBOARD LEADING EDGE SLAT RAGGED AND GETTING BIGGER. I ALSO OBSERVED THAT AS A SMALL PIECE OF ALUMINUM WOULD PEEL OFF; THERE WAS A DISTINCT BUFFET IN THE AIRPLANE DUE TO THE DISRUPTIVE FLOW OVER THE WING. I WENT BACK TO THE COCKPIT; RPTED WHAT I SAW TO THE CREW AND THEN CONTACTED MAINT CTL AND DISPATCH MYSELF TO ALLOW THE CAPT AND FO PREPARE A COURSE OF ACTION WITH A MOST LIKELY DIVERT. DISPATCH; MAINT CTL; MYSELF AND THE CREW AGREED TO DIVERT INTO ZZZ WHICH WAS NOW ABOUT 30 MINUTES AWAY. MAINT CTL BELIEVED THAT THERE SHOULD BE NO CONTROLLABILITY ISSUES UPON EXTENSION OF THE LEADING EDGE SLATS DURING APCH. THE 3 CREW MEMBERS IN THE COCKPIT ALONG WITH THE DEADHEADING CAPT AGREED UPON A COURSE OF ACTION WHICH INCLUDED STATIONING THE DEADHEADING CAPT IN THE WINDOW SEAT NEXT TO THE DAMAGED LEADING EDGE. HE RPTED VIA INTERPHONE TO THE COCKPIT AS TO WHAT HE WAS SEEING WITH THE SLAT EXTENSION. OTHER THAN A SMALL BUFFET WHEN FLAPS 1 WERE SELECTED; THE APCH AND LNDG WERE ALL NORMAL AND UNEVENTFUL.SUPPLEMENTAL INFO FROM ACN 753954: THE DEADHEADING PLT AND THE LCA MADE FOR FLAWLESS CLR AND TIMELY DECISION MAKING.CALLBACK CONVERSATION WITH RPTR ACN 753958 REVEALED THE FOLLOWING INFO: RPTR STATED A PORTION OF THE R INBOARD SLAT TRAILING EDGE DID IN FACT SEPARATE FROM THE SLAT AND WAS CONTINUING TO UNRAVEL AS THEY DIVERTED TO THE NEAREST ARPT.