Girls, advice pleas, how do i?

ok, this girl, she's 21., asked a friend of mine when we were hanging out if he was ticklish and he said no. she waited and later on snuck up and touched his ribs and he jumped a foot in the air. she said "oh, you were lying" and smiled. she had also asked me at the same time earlier if i was. i alsoo said no. i didn't think she could get my friend that easily though. and i am ticklish, especially in the ribs, right where she got my friend. if she tries, tell me some tips how to ignore it. she's good! and i don't want her to know i'm lying too. oh, help me!