once

Fielding once said . . . using once clearly places it in the past. Using with in the second blank is acceptable. I take it your friend is not a native speaker of English. I can't imagine anyone using argued unless there were a specific context of dispute--in that Fielding's remark is offered in rebuttal of someone else's claim. Of course you could also use wrote, if that were an appropriate verb (i.e., Fielding actually did write it).

I don't fully agree with Set's post because you can use the verb "argued" without there having been an actual argument. You can "argue" a case just by setting out the facts, and I think that this is what is being described here.

Helping with...or in...sexual harassment is an awkward phrase and needs to be expanded to bring out your actual intended meaning.
e.g. in the field of sexual harassment/ in cases of sexual harassment, etc.

I believe that Yeats has to work with the sentence having two blanks, and does not have the option to rewrite it.

Okay WBYeats, are you up to speed with this or do you want to give us a bit more text and context to work with?

Good answer. Thank you. If I find out more about the context, I'll post it up here. Thank you.