My daughter will be overstaying her visitor's visa at the end of the year and is planning to get married early next year. Can anyone please advise what she needs to do?

Not a good idea.. not all Americans are eligible to petition alien spouse. First of all make sure he does not owe tax, child support, restitution, etc. He has income 125% of the poverty guidelines, and does not have criminal record especially domestic violence.

From my experience, I would suggest that she returns to her home country and have her b/f filed for fiancee's visa. Immigration will screen the US citizen to ensure he meets the eligibility. It takes about 6 months but much safer for her.

In case the couple disagree and want to get married and obtain green card for her, since overstaying is in the picture, it's best to have immigration attorney handle the case.