There seems to be a somewhat hedonistic school of thought prevalent in threads like this that strongly suggests that significant others exist primarily to service our fetish "needs." Subsequently, whenever a case is presented in which a SO is unwilling for whatever reason, she is at the very least cast as incompatible, or more often cast as a pariah.
There is so much more to a relationship than fetish or sexual satisfaction. To base compatibility on such a shallow precept is a recipe for a doomed relationship, in my opinion.
She looks at tickling as a form of punishment, period. I've known girls like this. They rely heavily on tickling as a nonviolent punishment to even up a perceived imbalance of authority, or perhaps to create one in her favor. When he told her he liked it, that probably blew her away. Imagine how a mom would feel if her five year old son told her he likes it when she paddles him. Now Cyro's girlfriend knows that every time she thought she was punishing him, she was doing the exact opposite. Given the enormity of her mistake, it's completely understandable that she would stop tickling him, because it's now become a humiliating reminder of that mistake.
This is why it's seldom a good idea to blurt out to somebody - even somebody close - the extent to which tickling stimulates you sexually. No matter how much the TMF's Moral Minority lays guilt trips of "dishonesty, trickery, yadda yadda," it's nobody's business but your own.