A hypnotherapist (being a certified one myself) should be willing to work with various clients. The issue here is )the person who is receiving the hypnotherapy to be willing to work on the issues at hand. Hypnosis is not mind control and it works best when the client is open to the treatment (like w. any other therapy modality)
My suggestion is to chose a certified hypnotherapist with a Master's degree or above such as a PhD, PsyD, EDD etc. A Medical Doc is not per se required for this unless you have a desire for the provider to be a medical doc in order to address the brain injury component. If you already have a neurologist, see if you could get some good referrals for certified hypnotherapist in your area.
If in the USA, inquire from your mental health insurance carrier if they pay for this treatment (for instance Cigna pays for hypnotherapy) and if your insurance does in deed pays for this service, you may inquire about possible referrals in your area.
Other tips would be to contact the chosen hypnotherapist and see what areas they specialise in. In regard to the brain injury (my thoughts on this as that if the person receiving the hypnosis is functioning at a level of care that allows for this then there is really no "side effects" or risks that I can think of.
Again, hypnosis is never against one's will, it consists of providing verbal inductions to the client when he/she is in a relaxed state and thier brain waves function in Alpha level (which is a state that anyone experiences when relaxed, reading a book, day dreaming, prior to falling or waking up, etc)