My fiancé had overstay in us now she's living in the house of a married american guy. Can I sue them?

Why did she leave you? She has the right whether to get married or not. You don't have enough grounds to sue them. K-1 holder comes to US to get married, if you're not married, she must return to her home country within 90 days from the date she entered US.

She is not eligible to marry someone else or file adjustment of status. In plain word, if she married someone else, she must return to her home country first in order to file for green card.