The easiest way to find out if they used copper based meds is to ask them. If you aren't sure and they aren't sure then I would pass on the rock. I had some offered to me for free but the person couldn't remember if they had or hadn't used copper. I turned it down.
You will need to keep the rock in SW
with a power head. A skimmer wouldn't hurt either.
How long ago did they take their tank down? Has the rock been out of water?