I don't think "jealous" is the word.
More like "intimidated".
Cause, you know, Yau survived more days then she did, won more challenges then she did, is more loved by the fans tribe (which could turn him into a serious threat later on), got into 4th place out of 19 (better then Cirie's 4th out of 16), Yau would have won against anyone in the FTC.
Stuff like that.
It was the most logical move on Cirie's part.