Should I play it cool and let her come to me?

This girl has been dating some guy on and off for quite some time now. Everything I hear from her about this guy is that he's not assertive and self conscious and this and that and she just seems to only tell me bad things about him.

At the same time she's showing signs of interest, not consciously or purposefully flirting with me but I can tell she finds me attractive and we enjoy each others' company.

My question is, should I continue to lightly flirt with her but not ask her somewhere and just continue to let her come to me or should I make a move?

I wouldn't want to be the guy she is using for an ego boost and I don't know how she truly feels about this guy she's been dating (for about 4-5 months), so I don't know if I should try to get her attention or kind of neglect her until she figures out what she wants. If she's interested will she come to me?
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I don't know what the nature of their relationship is but if they've been dating for so long I'd imagine they're a couple, but she never refers to him as her boyfriend so I don't know what's up with those two
Should I play it cool and let her come to me?
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