Answer
(Fatwa: 557/L=38/TL/1429)
By not following ?iddah? if you mean that she does not observe the restrictions imposed by the Shariah (like going out of home, applying makeup etc) then she will be doing a great sin. However, if you mean that she gets married to any person during iddah then this act of her is unlawful and haram, their physical relation will be regarded adultery; such woman is asked first to complete her iddah and then get herself married.
Allah (Subhana Wa Ta’ala) knows Best
Darul Ifta,
Darul Uloom Deoband