Where F2 is the name of the Mentor whose Mentees you're looking up:
=IFERROR(INDEX($A$2:$A$4,MATCH(1,INDEX((COUNTIF($G$1:G1,$A$2:$A$4)=0)*(INDEX($B$2:$D$4,0,MATCH($F$2,$B$1:$D$1,0))="x"),),0)),"")