It's what we keep trying to figure out whenever our guests don't see our money requests.
We suppose that they get notified by email but we haven't ever received a money request as guests, so not 100% sure. And do guests read their email (linked with Airbnb account) or not?
It's difficult to know what they're doing with their device, once you know which device they're using.
One thing you can do also is to ask them to open this link:
https://www.airbnb.com/resolutions/
If they're logged in, they should see the money request there.