What you‘re doing for your friend is wonderful. I apologize if this post isn‘t concise, but I have a pounding headache.
With a traumatic brain injury, I‘d strongly encourage her to apply for permanent disability through the Social Security Administration immediately. You didn‘t describe exactly how severe the impairment or the prognosis, so I‘m going to presume it‘s severe and that she‘ll need long-term, possibly indefinite (lifetime) assistance that will increase over time. Hopefully, this isn‘t the case, but at least you‘ll have the info. Even if she already has health insurance, I‘d see if she can qualify for Medi-Care, or if your state has its own, similar program apply for that too, as she may need help the rest of her life.
I‘d contact any local volunteer agencies that deal with assisting disabled persons to get benefits: often if you call around to enough hospital patient‘s rights advocates, or legal aid places, they know of resources and agencies to refer you to to help her get everything she‘ll need, and I am presuming she‘ll need a lot. Some will even assist with filling out the paperwork. Sometimes there are volunteer agencies who help those with mental impairments specifically. A very good source of information would be if there is an ADA (short for the Americans with Disabilities Act of 1990) advice hotline in your area. I‘d call every Social Services agency, disability rights attorneys or entities, and even try contacting the Disability Rights Edcation and Advocay Fund for resources. Their website is http://www.dredf.org/ They won‘t be able to directly get involved, but can be a great place for referrals and suggestions.
You said she lived with her mother, and I don‘t know if the mother has a home that is paid off that she will eventually leave to your friend, but if not, eventually housing could become a serious matter. It may seem like overkill, but because the waiting lists are often several years long, I‘d have her apply for Section 8 or whatever Public Housing assistance is available in your state, if any. She may not have to move for a few years, and it may take her that long to get it, but some states offer housing vouchers so she could pick her own place to live at a very low cost.
Also, if she will eventually need in-home care assistance, doing all of the above will prepare her for that potentiality. There are also agencies that can handle managing her finances, if she‘s unable to, and noone else is available. Even if her mother or daughters help care for her/ eventually that could wear them down, so it‘s good to get the ball rolling ASAP, as it can take tremendous red tape and wrangling to line all the necessary benefits and assistance up, and dealing with the footwork, paperwork, and govt./state agencies can be challenging. I hope she won‘t need this much help, but better to err on the side of caution, because health problems can be catastrophic, especially over time, with unforseen consequences and expenses springing up, and preparing in advance is the best approach.
Again, I don‘t know the extent of her impairment, but if she is in any way mobility limited, getting her started through her health care provider on physical theapy could help prevent later complications from muscle atrophy and loss of bone density, greater risk of edema and stroke, etc. Health problems can set off a domino effect that starts cascading over the years.
Oh, and about transportation (I think you said she needs some), most States or counties have programs to help the disabled get to and from doctor‘s appointments, shopping, etc. She may eventually need to be moved to an area closer to public transportation, medical offices, shopping, etc. It‘s not easy to dig for all these resources, but she may need them even more as she ages. Also, even though she‘s still young, for later, consider that they do have "meals on wheels" that sometimes disabled people can qualify, or even foodstamps. She may not think she needs all this help now, but over time, any private funds she may have can be depleted, and the support system she has now may change. There‘s a big void that exists now that the sick, injured and elderly can fall into if a catastrophe happens and they don‘t have tons of money.
You are an amazing friend to be doing this for her. I‘ve learned through helping my mother that it took a very assertive (sometimes aggressive), and comprehensive approach, and I was lucky to help her get even just the "bare bones" minimum to survive. It took a few years and hundreds of telephone calls and most of the time I was sent in circles, so maybe this will spare your friend some of that. One final thought to keep in mind: if you run into too much "red tape" or get the "run around," don‘t be shy about going to your local assembly person and asking them to intervene. One phone call by them can sometimes spare months of waiting, and they are usually happy to help. I hope I haven‘t overwhelmed you too much with all of this, and best of luck to you both.