Real Estate law pertaining to jointly deeded property and disability of one of the owners

My name is Joe Coscarelly, I live in Marshall, MI USA. I am considering buying a foreclosure property to fix up and sell ("flip") with a friend. He is 77 years old and has a lot of construction skills to help me fix up the house. We are going to get a joint deed (Joint Tenancy With Full Rights of Survivorship). I just wondered what would happen if one of us became mentally disabled long-term. Would the other one on the deed have the right to sell the home in such a situation without the disabled parties consent? I talked to a local lawyer where I live and he suggested a Durable Power of Attorney but my friend is resisting this, saying it is an unnecessary expense. Just wondered if you could shed any light on this. Thank you. - Joe Coscarelly joecos@cablespeed.com