Paladin vs Thief

Paladin vs Thief

Paladin vs Thief

I have a Paladin player who asked, what happens if she uses “Pray for Guidance” (detect evil) and the Thief happens to be evil and has “Flexible Morals” (when someone asks for your alignment, tell them whatever you want)? 

Does the paladin’s ability detect evil alignments at all, or is it some other definition of evil that is used?