It's possible Harle thought she wouldn't be able on her own. Hence rely on Serge, who wouldn't need that much motivation to keep going after him.
Not to mention, didn't she had voiced concern about Serge pressing on into Lynx's scheme and wanted him to turn back? Unless that was more on the concern of Lynx furthering his plan rather for an actual concern to Serge himself. So perhaps she did had some genuine interest in Serge as a person. Well, at the very least, it kinda seemed like by the time she leaves the party there indeed was, but back in the early stages of the game...
