@Erin5717 I would advise you not to refund, regardless of how the guest left the place. It sets a bad precedent for guests to think they should get refunded, in contravention of the cancellation policy, just because they want that.
There seem to be a lot of guests who try this on with long term bookings. They book long term because it offers a discount, then cancel partway into the stay and demand to be refunded.
As far as the mess, I would just chalk this one up to a learning experience. If you are going to accept long term bookings, it is advisable to set some conditions that may up the price a bit, but ultimately be to your advantage, like a mandatory once a week quick cleaning, say half an hour, so you can make sure the guest isn't living like a pig in there, and keep the mess at bay if they are slobs, rather than letting it accumulate over the course of a month. You can couple this with a clean linen and towel drop-off and take away the dirty stuff to wash.
Guests will likely be more respectful of the space if they know you'll be coming in once a week and seeing first hand how they are living.