No, leave it alone - the wound will grow bark or it won't, but there's nothing you can add to it to help it do so. I would strongly suggest that you mulch around the tree, at least 3' wide, more if you can, no deeper than 4", and put no mulch in the couple of inches next to the trunk. That way, as well as keeping the soil cooler and keeping the moisture levels more even, the weed eater won't have to go near the trunk, and it won't get more damage. If grass or weeds do grow up next to the trunk, then hand pull them, don't use the weed eater.

Yeah mulching is a great idea!! It protects against both the lawnmower brushing against the tree and weed-eater damage.

String trimmers probably kill more trees than loggers.
A tree shouldn't be planted out in the lawn with grass growing up to the trunk in the first place......especially a fruit tree, considering all the chemicals that get put on a lawn. Bad for people and wildlife. I would incorporate the tree into a bed.

I forgot to mention that I mulched the tree right after it happened last year and cut out around it about 1.5 feet. I am glad to know that I should probably leave it alone now and not put anything on the part that is cut.