We did this last week. We chose an old friend of DPs who has spent a lot of time with the DCs and his long term girlfriend. Couldn't choose from parents as someone would be left out. Ditto my friends.
30 mins before the ceremony we were told the girlfriend had a vomiting bug. I phoned a friend, almost in tears, and she dropped everything and battled the traffic to be there on time, bless her beautiful heart.