If someone is interested in investing in rental property in a large city like Detroit, you should check out the demographic data at CityData.com .
Median family income is very low in Detroit, the unemployment rate is very high, and the crime rate is very high. If you plan on going all section 8, maybe things might work out, who knows, but that isn't for everybody.
A better bet might be a city like Indianapolis, which has housing that isn't as cheap but still very affordable, and much, much better demographics. Keep in mind that inner city landlording involves evictions (for non-payment, drug dealing, damages, moving in relatives, etc), repairs due to age of properties (old plumbing and other systems), typically higher crime, etc. You can make a lot of money in inner-city landlording or die trying, it takes a will of iron to overcome some of the problems you can be faced with. To top it off, you can be assaulted by people you show properties to, acquaintances of your tenants, etc. I speak from several decades of experience.