We have a coworker who is very into

Another coworker.
but they both are famales.
she is not sure how the other one thinks---what she really is sure is that lady protected her several times(argued with others for her severely), openly.
and that was why she began to like her.
that girl is openly a gay.
she said she cannot help thinking about her.
how can she do?
tell? Or maybe invite her for a lunch? Or, keep quiet.