Is she also bringing her own blankets, duvet, pillow protectors and mattress pad, or are you going to have to wash those when she checks out? Just figure out how much it would cost you to wash what she is bringing for the length of time she is staying and offer a discount accordingly. If you figure it's $5 for the time and hot water and soap, then offer a $5 discount. (I suspect she's angling for more, though).
But as Emiel says, you don't have to offer any discount if you don't want to. If her booking isn't already confirmed, and you are still in the negotiating stage, keep mind that guests who start out by asking for discounts are red flags. On the other hand, if she is staying long term, rather than a few nights, it will save you time and money to not have to offer clean linens every week.
If you don't want to discount, you could say something like" Some guests go through 3 rolls of toilet paper in a week, others only one, some guests drink up 8 cups of coffee a day and others don't drink coffee at all. Some guests use a lot of towels in a week, others only one. I can't really discount based on what a guest may or may not use, that would be a logistical nightmare."
@Earl7