To the best of my knowledge, there is no way to put it in an offline mode. But you could just set up a fixed schedule for them. Should the device go offline, I think that using fixed schedules it will still run.
If you don't mind monitoring it occasionally for them, you could set it up for them and make yourself an authorized user. That's what I did for my daughter who lives 1300 miles away from me. I actually set hers up on a flexible daily schedule, and it pretty much just runs itself. Hers is pretty simple -- just grass watering. Once I set it up, I look at it occasionally, but I haven't had to make any changes to it. And she loves it and how her grass looks now. And she says she loves laughing at the neighbors sprinklers running in the rain!
Yes, I know it's backwards -- I'm the "retired, elderly, parent" (OK, I am NOT owning the elderly part yet!!!) taking care of the daughter's system --- but I'm the techie and she's NOT!