Do the right thing and consult an immigration attorney who can review your past immigration history and properly advise you. DACA will not be impacted by the marriage, which in itself will give you the possibility of adjusting your status. However, your immigration history must be considered.
Priority date is the date immigration uses to determine whether is your turn to get the immigrant visa. Usually is the date a petition is filed with immigration. Consult an immigration lawyer in person for further information.
Your friend should contact an immigration lawyer ASAP. Generally when an appeal is denied by the Board of Immigration Appeals (BIA) the next option is to file an appeal with the US court of appeals for the 2nd Circuit ( if the individual hearing was in NYC), within 30 days of the BIA denial. However this appeal is very complex and your friend should contact an immigration attorney ASAP, who also can advise him on any possible Motions to reconsider or reopen.