Which tools do you recommend to buy when the person has arthritis?
Look for ergonomic tools with longer handles that are cushioned. Longer handles prevent the "digging" into the palm that short handled tools may create. Cushioning creates less tension and pressure on fingers.
Not all tools are ergonomic. How you use the tool will prevent the problems of misuse and overuse. For example, hold a steel hammer at the handle end. This will allow you to force a good blow to a piece of metal without the wrist suffering from shorter, less effective blows that cause unnecessary stress to the wrist.