This is an issue she feels strongly about and she doesn't care about letting her personal feelings get in the way of the whole thing.
In the Season 4 finale, Fiona tells Jesse that she belongs out there with Michael for better or worse.
She has been shown to be so highly effective at this, that she surpasses even Michael Westen.
This is especially true at places like amusement parks, night clubs and concerts or even just pushing around the grocery cart! Michael turns to a familiar arms dealer for help in tracking down the bomber, and decides to help out a woman track down her dead artist father's stolen painting.
Fiona waits until Lydia to make a visit to the restroom and catches the older man making a move on an even younger girl.
After tracking Lydia to the carnival, Fiona is disturbed not only by the choice of dress but by the fact that Lydia is meeting a man much older than herself.