I think, in this sentence, "their" should be "his" or "his or her", because the subject of the sentence is WHOEVER and it should be treated as singular. ), but how do average native speakers actually feel when they see this kind of lack of agreement? I cannot speak for the "average native speaker" because I haven't met them.
New words, one handy idiom, and a 2-minute quiz — delivered to your inbox to keep your streak alive.