Honestly, you are trying too hard to defend her, Themrys. Any kind of information that may be inconvenient to the other partner is expected to be disclosed. Being legally married implies other potential consequences/dangers that Durkon may prefer to avoid (retaliation from the husband family/clan, for example). It's not everything about Hilgya's feelings. In fact, Hilgya's situation is inconsequential to the fact that she lied*.
She asked if Durkon was married. So she knows people have a problem with people who are legally married. Her pretending that she isn't legally married doesn't make her any less of a liar. And to Durkon no less, who she is supposed to having "fell in love". She was in the wrongs here, and the only "sin" Durkon committed is that he was naive enough to trust her. Admittedly, it's a minor infraction, but was significant enough for Durkon to react in the way he did. If he had knew, he wouldn't have consented sex. She manipulated him.
*Which I think would be different if the husband was actually abusive and did so terrible things to her that she prefers to pretend he never existed. But we know that this is not the case, and that Hilgya is more a rebellious child than a victim of a dwarven patriarchy. For all we know, the arrangement should have been merely political and probably the husband was equally coerced, but isn't as rebellious as her