1) Don’t offer to hang out whenever. Instead try to be specific, schedule a time for the two of you to hang out and do something that isn't writing related. (Yes, even if you’re their significant other.) Having an event planned for the evening might add the right amount of oomph to help the writer get everything done earlier in the day.
2) Don’t be annoyed if they cancel a plan. Writers put a lot of stress and pressure on themselves. If they don’t think they have the time to hang out please be assured that it’s because they’re worried about their work and deadlines. Not because they don’t want to hang out with you.
3) Do feed and water them. When a writer is in the middle of a great new idea they can forget to eat, drink, and sleep. Helping them by putting food and water (or coffee) on their desk will be greatly appreciated, though they might not show it until they’re out of their creative fog.
4) Do show interest in their work, but don’t pester them. Knowing that your there to be a kind and constructive beta reader is really helpful. But a writer usually knows when they are ready to show their work, so don’t push too hard!
5) Please remember to comment on both the bad AND the good. This applies to how they act during writers block just as much it applies to their writing. Thank them for washing the dishes and ask if there is any way to help them through their current predicament. Leave comments saying that you were laughing out loud and highlight sections that were confusing. Most importantly, be kind and truthful.