What do they need to get besides death certificate from fiancé's previous husband and how long do they need to be here prior to ceremony?

Marry and then what? Apply for the spouse's green card? They cannot just come to the US as a tourist with the immigrant intent to stay.

Along with the required forms and documents for a relative petition and application for adjustment of status, your friend will need to submit her marriage certificate, the death certificate of her previous husband, her proof of US citizenship, an employment letter/verification, her latest tax return, 2 photos of herself, and evidence of a good faith marriage. In addition, her fiancee will also need to submit proof of legal entry into the US, her birth certificate, a medical exam conducted by a USCIS approved civil surgeon, and 4 photos of herself. The forms should not be submitted earlier than 61 days after the fiancee enters the US or the government will presume that the fiancee committed visa fraud because she had the preconceived intent to immigrate to the US permanently when she entered the country.

If same sex marriage is recognized in the US state or in other jurisdiction where the same sex couple intend to marry, then, it is suggested that the couple get married in that state. With regards to the length of stay necessary to enter into marriage in a particular state, this will depend on the residency law of that state. Please contact the clerk's office in the country of the state where the couple intend to marry for this and other requirements. A death certificate of the previous fianc?e will not be necessary nor required if the ex was not in fact legally married.