I'm so sorry to hear about Dolly. It's a sad thing because their cage mates will indeed mourn. When our Snowball died the other two budgies kept calling for her and flying around. All they knew was that she was gone. It took them a couple of weeks to finally stop calling for her. They actually kept looking out the window as if she was out there.
It would be nice to get Budgie a friend, especially if he's been used to having company.
It's hard to lose a special friend.
Rest in Peace Dolly.