Why would an amputee never be able to use a prosthesis?

Tojay, without being  critical, this sentence reveals a lot:

"I have heard of some amputees never being able to use a prosthesis, but not the reasons. "

Gossip, word of mouth, "hearing of" and other potentially unreliable comments won't help you or your housemate.  Nor will concern about reasons why someone couldn't successfully use a prosthesis.   And those reasons may apply only to those individuals, Conversations will knowledgeable people treating your roommate could though.  

And as others have observed, there could be physical issues that interfere with or preclude adaptation to using a prosthesis.  

If you want to "hear of" amputees who are successful, read some of the literature and posts about military amputees who ARE successful.  One you can start with is Sen. Tammy Duckworth, who lost both her legs when the chopper she was flying was shot down.

https://www.duckworth.senate.gov/about-tammy/biography

The second paragraph describes how she was injured.  She didn't let it "keep her down."    She ran and was elected to the Senate.   She's a brave woman, as are others (male and female) who move on with their lives despite amputations. Let someone like that inspire you and your friend, and don't rely on sources that aren't explained as to rationale.

Here's another brave woman:
https://www.wpri.com/news/local-news/west-bay/state-senator-recovering-after-doctors-had-to-amputate-her-leg/

You wrote:

"She’s counting on this being just another bump in the road to getting her life back. And totally counting on being able to use a prosthesis and walk again."

Good for her!  I admire someone with that attitude; it makes a big difference in how she adapts to the prosthesis.

Support her all you can, with positive information and reinforcement.  She has a brave and strong attitude.   Some folks don't, and that's not a criticism; amputation is most definitely a hard situation to face.

During one of my parents' rehabs, we met another patient, a diabetic, who had just had one leg amputated below the knee.   We became acquainted with the family, who were so frustrated b/c their parent refused to eat diabetic foods, but continued eating whatever he wanted.   In the space of a few months, the other leg was amputated below the knee.    

Attitude and compliance are important; your friend seems to have this approach.  I hope her future life reflects this attitude, for both of you.  

Good luck to your friend, and best wishes to both of you in this challenging time.

P.S.  You might want to search online to see if you can find a forum for patients with prostheses, or check out one of the VA sites on this issue.  I'm sure there will be a lot of discussions on adaptation on those sites.

My cousin injured herself in a skydiving accident many years ago. The resulting surgeries to repair her ankle/lower leg left her in excruciating pain 24/7.

After years of revisions, rehab and never healing well, she opted for a below the knee amputation. Due to her diabetes and weight problems, a prosthesis never worked for her. For a time she had a metal 'leg' and that seemed to be the best answer, however, her diabetes and weight played against her. She opted for wheelchair life at the age of 45.

I am not close to her, so I just hear about how she's doing. She is happier in a wheelchair, so perhaps that is an option for some people who cannot adapt to protheses.

So many factors contribute to this dynamic. What works for one is not standard.

There may be many reasons as each patient is very individual. They may be due to nerve pain in the limb, to poor healing and pressure at the end of the limb, to balance, to the quality of the prosthesis, and on and on and on. No reason now to jump ahead to this. Try to take this more a day at a time. Some patients have problems with healing chronically and need stump revisions. So you just have to take things as they go and hope for good rehab and PT help. Sure wish you the best. A weakened heart helps absolutely NOTHING. There is more difficulty with breathing and with swelling of the extremities with heart failure, dependent on whether it is right sided or left. Hopefully the stents will help with circulation. But CHF means congestive heart failure, and the heart being that pump that everything else is dependent on, any "failure" in it has consequences for every single other system in the body. But many with a perfectly healthy heart have difficulty with many things relating to an amputated limb, all the more so if there is any diabetes and poor healing involved. Hope for the best and try to take it day at a time; wishing you the best.

My GF was a juvenile diabetic. She could wear one because she continued to break out in sores on her stump. A prosthesis would just make the sores worse. Have no idea why my other friend chose not to use a prosthesis. Don't think they are easy to get used to.

First Google search I came across (so not actual reputable medical advice) states prosthesis for diabetics differ. I wonder if due to extra needs surrounding infection & preventing tissue damage.

"Many people with diabetes who have had a limb amputated opt for a prosthetic limb replacement. Prosthetic limbs for individuals with diabetes are designed, fitted and maintained differently than prosthetic limbs for people who aren't living with diabetes".

Of course your friend will need specific medical information for him/her.
I hope a prosthesis works out.