Elder Financial Abuse?

My Dad is showing signs of dementia, although he doesn't have an official diagnosis. He has some friends that he tagged along with to a car dealership. He "doesn't remember" what his involvement was in their purchase plan with the car dealership they visited but he provided his social. My brother is his caretaker and received a bank statement from a credit union my Dad never had an account with until his filed trip with his neighbors. I believe he cosigned for a car loan for his neighbor. This is not a close friend or family! I believe they took advantage of my Dad. He's a giving human and loves attention and admiration. He's isn't of sound mind to Make financial decisions and he also doesn't have any assets that could be seized if the neighbors default. Do we need obtain a lawyer? My brother has reached out to the car dealership and bank and they won't give him any additional information about what my Dad contributed too.