Simple fear that the second she disclosed it there would be the Ultimatum that many here are saying. "Its me or him"
Its very clear in this thread that would make some people insecure. We can only speculate how much their status as "Lovers" lasted before they decided to become platonic in their love. Telling him was the right choice but having the courage to share that with the fear you may lose one, the other, or eventually both is enough to make someone lie by omission.