Is my window air conditioner cooling properly?
According to this, the Frigidaire FAS182H2A1 is an 18000 BTU device.
http://www.appliancepartsnationwide.com/FAS182H2A1/53279.htm

That's pretty powerful if working properly. The government suggests 8000 BTU for 325 SF.

http://www.energystar.gov/index.cfm?c=roomac.pr_properly_sized

Check your outside coils too and clean if needed. If that is not a problem it could be something internal. Some examples:


partial clog in your expansion valve.  
low freon level


If cleaning the outside coils doesn't work, I do not expect repair to be practical. You can buy a new 10,000 BTU unit from Home Depot for $300. It would be reasonable to ask the owner to pay for it.