Lets unpack what this means for a second. Does it mean he raped her? Does it mean he coerced her into sex? Certainly that, or something similar, might be what is implied by this statement. But what really happened is she had sex with him, but didn't get anything else in return from him.
That's because, for a lot of women, sex is meant to be transactional. It's not supposed to be something that happens because you enjoy it and because you want to share that experience with another person. Sex is something that she gives to the man, often in exchange for the man giving her something extra in return. He isn't meant to get it for "free".
And when this happens, she says he took advantage of her. Maybe he lead her on. Maybe he said he had more money than he really does. Or maybe she was horny and decided to peruse sex before establishing what it is that he should give her in exchange. But at the end of the day, she didn't get what she felt she deserved from him. And as a result, she thinks he took advantage of her.
And let me tell, this is a toxic, nasty attitude to have. I think this lies at the root of many of our gender dynamics, and is a fundamental, driving force behind how men and women interact with each other.
Is there something similar that happens in reverse for men? I don't know, I figure I'll ask that question here for anyone who wants to answer it. I don't want to point fingers, I just think the phrase, "he took advantage of her", is a loaded, dishonest expression. All it underlines is the assumption that women refuse to have sex on mutual, equal grounds, with men.