Nasty topic.
I think it really depends on the context here.
If you've taken all possible measures, e.g. treated your listing regularly with pest poison and tried to limit unventilated moist area in your place and if the listing is situated in a tropical area where they are endemic and out of your control, there isn't much else you can do.
If your listing is indeed situated in a tropical area where this happens regularly, I would consider briefly mentioning in the ad the possibility of occasional tropical insects. To be upfront about it and it'll give you more of a leg to stand on if/when such a thing happens.
Now if your listing is an appartement in the middle of a city and you attracted cockroaches because you failed to instal a simple ventilation in the bathroon thus creating an unsanitary and artificial tropical zone, that's a different story...
In any case, complete, partial or no refund I would really encourage you to treat the guest very gently and with a lot of understanding. Especially is they're not aggressive about it and can document their claim.
I went through a similar situation myself as a guest and I can tell you it can be a very traumatic experience. Seeing them creeping aroung you and having to sleep in a bed in which you saw some running around just moments before is no small feat.
They are very dirty (bacteria), can bite and lay eggs everywhere. The phobia to not get rid of them and bring them home with you is real. I had one of them jumping out of my luggage at the airport...
So yeah, to the hosts out there, do not get defensive too quickly and dismiss this horrible experience too lightly even if people might freak out a bit, especially at first. Take precautions to ensure the risk is as low as possible.
To the guests, keep in mind that in some parts of the world the risk is real, no matter what the host does and that it's (unfortunately) part of the experience in such places.
Last reaction to some comments below:
No, dirt and food rests won't "create" a cockroach problem, anywhere. They spawn from warm and humid corners: e.g. pools of stagnant water.
Dirt and food rests will attract them, if they are already present in the area.
Peace out and keep traveling the world !