Its a difficult one - and a situation I am dreading as my two are now 13 and nearly 11. And extremely close. When Lovebird was run over, Donner never did get over it completely. He was pack alpha but ruled from a distance. He never joined in with the others in games or hunts, he was and remained a loner for the next 12 years. They were two when she was killed.
Since then the pack has grieved whenever Rainbow Bridge time called for one of their number but because they were always a pack - they helped each other get over their loss. Being down to the smallest pack I've ever had is going to present problems with the one who is left, I'm sure.
A new puppy - fast - might be a solution.