I am having the same problem. When I go to do the side mission nothing appears on my map to go to. Even when I go to my journal to active the mission, still nothing pops up. Help would be appreciated, do know if anyone else has this problem.
I've had similar issues on a couple of missions. The only way I've found to get round it is to load up an earlier save and work to the same point. Hope your last save isn't too far back and sorry if it doesn't work for this mission. Maybe you 2 could work out which one of you has the closest save, then only 1 of you has to do it and if it doesn't work the minimum effort has been expended.
If people could like this comment it would be nice. I have bugs to report but can't post new threads because I'm new and need points.