Ok, I found the information tucked away in one of my recipe binders.

1 cup honey to 3/4 cup maple syrup.

It does not say anything about adding more sugar or accounting for water, but I assume the lesser amount takes care of that.

I used this conversion in creating a maple cookie recipe with no butter from a honey cookie recipe. The honey cookie used 4 Tbs., and I used 3 Tbs. maple syrup. I also added 2 Tbs. maple sugar and 1 Tbs. granulated sugar, maybe because I wanted them sweeter. I also used 1/8 tsp. maple extract to up the flavor.

As for why someone might want the substitution, some vegans do not eat honey, although challah has eggs, so clearly this person is not a vegan. Perhaps there is an allergy?