My partner works in a shop that consistently lets the temperature dip well below a reasonable level. Tonight, it fell below sixty degrees (outside temperature was forty-seven degrees) inside, and the employees don't have a way to adjust the thermostat, as it is locked up.

I know there are laws governing a workplace environment, but are there laws relating to the conditions that employees are forced to work in? While I find it unreasonable, the law is an entirely different matter.

OSHA has issued guidelines with regards to reasonable temperatures, but they are just that - guidelines, not regulations.

cbg is correct and here's a fairly recent interpetation publication for you to read to verify;

http://www.osha.gov/pls/oshaweb/owadisp.show_document?p_table=INTERPRETATIONS&p_id=24602