how far can one turn down a crank?

Just need to take it to a shop and have them check it for cracks. Sometimes the cranks will crack if they get hot enough to turn blue. If it isn't cracked then the shop can turn it down until it is good again. You should be able to get undersize bearings without too much problem. Not really a big deal since the guy gave you the crank. If you can fix it then you're good to go, if not then you aren't out much.