AFriend,
Thanks for your committment to and interest in your friend. You ask a very complicated question.
First of all, is your friend seeing a doc that you and family/friends have confidence in? Is your friend seeing a neurologist? A movement disorder specialist (MDS - a neuro who specializes in movement disorders such as PD or Huntington's)? If not, I would encourage your friend to see an MDS.
DBS? There is a variety of criteria to meet for someone to be eligible for DBS. I do not proclaim to be an expert on DBS. I do understand and have witnessed what DBS can and cannot do. It's an individual, educated decision that one needs to make. I will defer to any DBS expert. Most DBS hardware it made by Medtronic. I imagine they have a website. Imagine it might be banned if I mentioned the company name. (Ed, moderator, will let me know.)
Why don't more folks avail themselves of DBS? It's brain surgery. There are potential side effects - some of which the experts don't seem to share. Ex., folks who have had DBS sometimes (a relative term) have issues with speech. There are different philsophies on this. There always is risks associated with any surgery.
Some symptoms are addressed better by DBS than others. Ex., balance issues are generally not addressed by DBS. Tremor is.
I have had PD symptoms for 12-13 years. I have not had DBS, nor am I a contender at this point. I have my opinion about DBS, but would prefer that those closer to the issue offer their comments. Above all, your friend and folks in your friend's life should educate themselves on PD, meds, alternative therapies, and DBS. Your friend is fortunate to have the concerns that you raise. I hope someone with DBS expericnce will respond. Else I would start a search and get feedback from others.
Best of luck to you and your friend,
P.S. I think Medtronic has a site that will offer info on the procedures with their hardware. I hope to stimulate your curiousity and to assist your friend in finding the best alternative.