The filmmakers had to complete the movie without him, and they used a combination of CGI, body doubles, and unused footage to finish the character of Brian O'Conner's storyline. They also worked with Walker's brothers, Cody and Caleb, to stand in for him in some scenes.
So while Paul Walker is not physically present in Furious 7, his character Brian O'Conner was able to be featured in the movie in a way that honored his memory and brought the character's story to a fitting conclusion.