Apparently it is confirmed that it used to, however there is a deal of talk that it no longer works. Whats the deal? Did riot hotfix this? Was it a bug or an intended mechanic?
There is no mention by a Red it ever did, and as I don't have kha'zix and even if I did I prolly couldn't get a pentakill I can't give you a confirmed answer.
I hate to use youtube as a source but here is your pentakill to evo point.
The only RED confirmed way that this is accomplished is listed below. Rengar and Kha'zix interaction : If Rengar kills Kha'zix after a certain point (level 16 I think) he gets the "Head of Kha'zix" and can't lose stacks anymore from his necklace. If Kha'zix wins he gets a 4th evolutionary point letting him have all 4 evolutions.