Hey Samuel, I live and work as a real estate agent in the North West valley so I am very familiar with the area. On a positive note he has good tenants, I'm assuming it's a newer home since most of the homes in that area were built after 2005 (So low maintenance) and the properties are local so they are easier to manage. You are absolutely correct that you could get better cash flow in other states but with that he would likely hire a property management, Need to find need good tenants and if he purchased an older home it could have it's own list of problems.
I don't have a crystal ball but I also agree a correction is coming in 1-2 years but not just in Phoenix, it will be nation wide. I would not suggest selling his homes in AZ to invest in another state just for appreciation reasons but it could make sense if the extra cash flow makes it worth it. I would just make sure you are familiar with the other areas you are thinking about investing in.
I know you mentioned multiple mortgages and current market values but you didn't disclose how much equity he has in the properties and how much the mortgages are for. If he has a lot of equity maybe he can sell one property and pay off the other 2? If he is getting close to retirement he probably wants to play it safe and he doesn't want to be over leveraged when the market does correct it's self.
My suggestion if he has a lot of equity would be to sell one property to pay off the other 2, that way he no longer making mortgage payments and can increase cash flow well playing it safe. However if he isn't able to do that it could be a better option to sell all 3 properties combine the equity to buy 1-2 properties for cash in a more affordable state that offers higher rents.