Morisj, I'm sorry to hear about your father-in-law's diagnosis. I was diagnosed in 2001. There certainly are worse things. We have to hope that all the research going on will bring some positive results and ultimately, a cure.
I have not started on an anti-depressant, but have many friends who are. The rate of depression is higher among folks with PD than the average person ... which is to be expected. Getting the right anti-depressant and dosage can take awhile, from what I understand. The individual should work closely with their doctor/neuro until they do get it right. Depression is a nasty animal. Don't let it rule your friend or family-member's life. It can happen ... but doesn't need to.
Best of luck! Your father-in-law is lucky to have someone (you) so interested in his well-being and quality of life.