If applied to the leaves of the desired plant, the only safe way is to remove the infected leaves ASAP. The plant may look bedragled and ugly, but it may survive to reflourish.I suppose you could quickly hose off the weed killer, but I suspect its too late for that. If the weed killer is applied through the soil, no chance to save it, sorry.
Depends if the herbacide kills from the leaf down ,or from the root up.Either way it's a bit late now with all the hot weather we're having.So its off to the plant nursery you go!