In order to help Robinfur, you must become a medicine cat (EXP MEDICINE CAT OR CAN JUST KILL AL THE CAVE GUARDS) then receive a prophecy, talk to Robinfur and sleep. You will enter this creepy nightmare but stay calm, all you have to do is find all the Firestar shadows. I think one is in the leader den, the place where Plumclaw does patrol and the last one just go the the left. When you wake up, talk to Robinfur again and then you can mate with him/her.