A friend's been sick and had not been using his cast iron grill/griddle for approximately 8 years. He decided to give them to me. The grill/griddle was seasoned (with oil) 8 years ago, but the oil on them is now sticky to the touch. If I don't want to get food poisoning, do I need to wash off the old oil with soap? or is it safe to start using the grill/griddle as is? I've attached a picture for reference.
It won't give you food poisoning, but re-seasoning (rather burning off clean and then immediately re-seasoning) is advisable if sticky (which means, IME, it was oiled and put away 8 years ago, not actually seasoned (heating to polymerize the oil) 8 years ago.
The canonical seasoning/re-seasoning cast iron thread here: https://cooking.stackexchange.com/a/647/34242
You can remove the ancient oil with something alkaline.
If you put it in a plastic bag with ammonia and tie it up for a few days, when you take it out the old grease will come right off with water. Other option is to leave automatic dishwashing detergent (with sodium carbonate) or washing soda (with sodium carbonate) on it for a few days. Ammonia and sodium carbonate are bases and saponify the old oil which makes it much more water soluble and easy to remove without serious scrubbing. It is less smoky and more complete than burning it all off. It makes it come off easier than just soap.
Getting rid of the old oil gets rid of rancid grease taste risk. Then you will need to season the metal as with a new grill.