Hi Steve, I'm not an official expert, but I do dabble with dryer vents both for fun (weird, I know) and in my line of work (plumbing, air, electric).I hope these help:1) 90 degree bends are pretty bad to have because they restrict air flow.

To allow for air flow. 

If the unit has good air flow, such as you would find with a forced air HVAC system, it will lessen the chance of moisture build up.If the tenant had blocked the sliding glass door and surrounding area with excessive furniture, boxes, or clutter, then that could lead to moisture getting trapped.

This could reduce the air flow at the vent drastically while still allowing just enough flow at the vent to feel the cool air.

That said, if the asbestos is not friable and/or in an area accessible to the tenants (or from which airflow can reach the living space) and can be contained (i.e. pipe insulation in the cellar and walls that can be easily encapsulated), then you would be fine simply encapsulating it.

I am rehabbing a house in Silver Spring MD right now, and there was a vent near the furnace, which I thought was to bring more airflow for combustion.

Opening the basement windows provided some airflow but did not reduce humidity at floor level. 

During his visit he fully inspected the AC unit, testing air flow, inspecting coolant levels (which were found to be normal - the tenant had complained that coolant was leaking, that turned out to be condensation) and establishing that the unit is blowing cold air at a normal rate.

This will keep your equipment in good shape, make it last longer, will uncover potential problems, and may potentially keep you from a costly night/week-end call-out.At minimum, this check should include about 20 or more items, checking the capacitor, amp draws of your fan, compressor, pressure check, air flow check, inspection of electrical components for corrosion/pitting, and many other things. 

I point out that the rental agreement requires them to maintain the temperature in the whole apartment or house at no less than 55 degrees and air flow is important to keep things dry.