Cagemate death/Depression

Question
I will try to make this as short as possible, I tend to pay too much attention to detail. First let me begin by saying THANK YOU for your time and services, it is VERY hard to find an actual person who can answer questions about our wonderful pets!

The begin, my husband and I had two ferts. One, Oliver, was turning one in December and we have had him since he was a kit. He has recently passed on. The other fert in question is our five month old Bella. Who seems to be taking the death of her "brother" poorly. Oliver, passed away two days ago, from Kidney failure. The vet assured us it was NOT viral or bacterial or anything we did or didn't do. That it was hereditary and there was no reason for his illness. We had been dealing with his illness for about two weeks until the last visit, when they found out his kidneys had begun to fail. The two weeks up to that point he was extremely sick, off his food, had diarrhea (ranging from bloody, mucus, to green) and vomiting. He was on several medications and continued to get worse, it was a very difficult time. We did allot of online research and thought he possibly had ECE, but Bella was showing NO signs of distress in anyway, and we have had them both with no outside fert contact for 5 months(since we get Bella) Toward the end, she began to act as though she understood he was very sick and began to sleep with him on the floor of the cage (he was too sick to climb to the 4th floor where he normally slept.) Again, the vet assured us there was no way he could get her sick, but also mentioned after we had to put him down she may handle his death poorly.

Since his passing, she has been acting differently. Not to the point I would say was was defiantly depressed, but different. She is not eating normally, so I am trying to entice her with lots of snacks, which she normally takes, but still not much food. I've cut her water with pedilite to again entice her, but shes still not drinking enough. We are letting her out of the cage whenever possible. We are very lucky, my husband and I have crazy work schedules that allow us to spend more time at home then most. I would say she has been out of the cage approx. 10 hours a day, were smothering her with attention and have bought several new toys for her. When she is out of the cage, she acts normally, but when we put her in the cage, she acts as though she is looking for Oliver, then immediately curls up in his old blanket and sleeps. I understand ferts normally sleep allot, maybe I am just being overconcerned, but I feel she is not acting normally. Also, her poop is now turning a green color as well? It is still normal texture (Olivers was not) and she is not acting sick in any way. The vet has been contacted and again assured us there is no way he got her sick. She thinks Bella my just be stressed and that is causing the color change.

The question is, should we concider adopting a new friend for her or see if she can make it through. Even though the vet had assured us she is not sick, the thought of getting another Fert is scary, for more then one reason. Its just a painful thought for us in general, and what if she is sick? or the new fert is sick? I've read that its a bad idea to take a middle age fert and get a kit, but she is only 5 months and still very active. However, we would prefer to adopt then get a new baby. We do know someone who has a son with two ferts that does not want them anymore. They get no attention or love. However tempting to "save" them, it seems like a bad idea to me to get two ferts with developed relationships (they are both one and a half) and add them to a young fert who may be dealing with minor depression issues? I'm at a loss for what to do? Should we see if she can make it through, and if we do will she ever be fully happy again alone? Is it a good or a bad idea to try to introduce two slightly older ferts that already have a relationship?

I hope I've given a general idea, sorry if I've jumped around, allot has happened in the past two weeks.  :( Thanks for your time and help!

Sincerely,
Brianne and Brandon

Answer
My first thought is did you get a necropsy?  One is very young and if she is showing some symptoms, then I would be concerned.  ECE isn't as common any more because Marshall ferrets seem to come with an immunity now, but something seems to be wrong.  I wouldn't add a new ferret until your existing one is healthy, completely healthy.  We can bring things home to our ferrets so it may not be something your vet is looking for.  I may also recommend a second opinion.
Once she is healthy, it is usually no problem to introduce another ferret.  I haven't noticed ferrets mourn really (although others would probably disagree) but it doesn't mean that she isn't used to another ferret and is not alone.  I have mixed all kinds of ages to my group.  
If you do get the other 2, keep them separate for 14 days to make sure nothing is contagious.  
Green poop and lethargy doesn't sound like depression, it sounds like an illness and I would either force the  vet to take it seriously or try another vet.
I hope this helps.