People who are prone to panic often find they are able to function more easily if a trusted person is with them. Left to their own devices a panic sufferer, who quite possibly experiences agoraphobia too, may be reluctant to leave the security of their home. Those with less severe symptoms may be able to get around very familiar routes, but have great concerns about venturing further unless accompanied by someone they trust.
In many ways this places the supporting person in a unique situation. They may feel the burden of responsibility and/or flattered by the trust being afforded to them. They may find some days easier than others and some situations more taxing to cope with. They may be puzzled as to why the person they support appears capable of functioning with but not without them. Here are a few tips to help things along.
Keep in mind that anyone who experiences high levels of anxiety tends to have good and bad days. The fact that you were able to go around the shops yesterday does not necessarily mean you'll be able to today. It may be frustrating but try not to provoke the person by pointing out obvious discrepancies in their behavior. It's almost inevitable they already know and there's a danger they will feel even further embarrassed and self-conscious and use this as a reason to withdraw more.
Panic can hit quickly. You may find yourself strolling along only to find the person with you suddenly comes to a halt and is beginning to struggle with their breathing. They probably look extremely alarmed and may start to shake, stagger and reach out for something to grab hold of. In situations like this the person is feeling very unsafe and extremely insecure. Assuming both you and they realize this is a panic event and not a genuine medical emergency, you can help by physically supporting the person - just holding the arm will probably do. Remind them this has happened before, that they will be alright and to remember how to breathe (see three breathing techniques for anxiety).
If things are going well, don't press the person to do more than they want, or assume they are capable of more. In most cases if the person feels up to it, they will suggest the course of action.
Take what the person says to you at face value. If they say they need to go, they mean it. If they ask not be left alone, don't leave them. Specific requests like this occur when the person feels vulnerable. Remember, they are probably already extending their usual boundaries by being with you and it may not take much to tip the balance.
Up to now my tips have put the onus of responsibility on you as the person giving support. It would be unfair and unreasonable to suggest you carry a burden all the time. There may be occasions when things go wrong and there's really nothing you can do about them. It's quite possible that despite all your care and precautions the person may still have a panic attack. This isn't your responsibility. You are doing what you can to give support and that's all.