That person, not you, knows what feels most comfortable. Eventually you don't have to be present all the time. Don't wear yourself out. You may not understand panic attacks, but never tell the person that it's all in her or his head, that he or she could go out if she or he really wanted to. PA's and anxiety don't work that way.
Every outing is successful if looked at correctly.
As part of your support role you may have to remind the person that backsliding is normal, assure them that they are sane and that they are not having a heart attack or other physical trauma. The person may be very up-tight. The person is probably anxious, and to plan as though you were preparing an invasion will make him or her more anxious. How much planning and structure is required varies from person to person and will probably change over time. If you are not familiar with the place you plan to go, go ahead of time to case it out.