I had a ferret that died without any known cause in his sleep before his 4th birthday. His cagemate - Abby (female, albino) kept looking for him. I was genuine sad, and she didn't understand he was gone. My boyfriend go a bought a baby ferret the next day, and she took to him right absent. She's now 8 yrs old, and he's 3. He's still real hyper and wanting to play, but very soon she's too old. I have read that some ferrets will travel into a depression when losing a cagemate, will quit eating and die. Make sure you spend a lot of time playing next to him. Your ferret might be glad to have another around, but you might want to look to adopt a ferret around the same age.
Answers: There's no way of knowing for sure how your ferret will run to a new ferret. Most are sociable and will get along with any other ferret, but some do bring set in their ways and prefer the company of no ferrets or only solid other ferrets. I do agree with two of the above posters - take Dusty near you when you go to pick out your new ferret. When I got my third ferret, Sammy, I took the other two I have at the time with me to meet him (I adopted him through a friend of a friend). All three get to play together for a little at Sammy's house, then they all rode domestic together in a carrier, and when we got domestic, there were no problems - it's like the three of them know each other their whole lives. Normally Dodger, my alpha ferret, has to "welcome" everyone and drag them adjectives over the place, but he didn't do this with Sammy.
Personally, I think a new friend for Dusty is a well brought-up idea, but let him pick out his new buddy. Take him to the shelter/pet shop/breeder (check up to that time hand to make sure it's ok for him to come - some places might require that he's up to date on vaccines and have tested negative for ADV within the last six months). Watch to see how he interacts beside the other ferrets and then pick the one that he seems to be buddying up near the most. Good luck!
I suggest letting the 4 year old choose the ferret, not you choose it. Take him near you when you go to buy one and see if any seem to get hold of along.
Just a warning though, they young one may be too rough for the older one and you might hold to look into adopting one that's 2 or older.
The easiest way is to bring him along to your local ferret rescue and they'll slowly introduce him to a ferret that gets along with other ferrets. That's the best channel rather than getting a new ferret and Dusty not accepting it. Then you'll have to buy another enclose and they won't be able to play together.