Citronella is bad stuff! Oil in wood is never good. Kitty litter would be my first step, but anything caustic will damage the wood if left on too long so the follow up to getting oil out of concrete won’t work.
If it were me I’d try the kitty litter and then shoot it with some rubbing alcohol, since that will remove some of the oil…but really they just need to buy a table cloth and clip it on there 
sorry
