Welcome to the Disney Parks Moms Panel, Cecilia!
I'm sorry to hear that one of your friends isn't able to join you for your trip to the Disneyland Resort, but glad that another friend will be taking his place!
Pixar Fest, happening now through September 3, 2018, is all about celebrating friendship, so you are certainly embodying this theme! As long as the theme park ticket hasn't been used or activated by the first friend, your other friend should be able to use the ticket. You won't be able to change the name of the ticket in the
Disneyland app, however.
When you arrive at the park gates, the tickets in the app will be activated and you will be issued paper tickets to use as well. At that time, a photo of each person will be taken and assigned to the ticket. After that point, the ticket is non-transferable. It might be a little confusing when using the ticket in the app since it will have a different name, but otherwise your friend shouldn't experience any other issues when using the ticket.
I hope this helps. Have a magnificent time at the Happiest Place on Earth!
-Linda