What is the best way to support someone with panic disorder tendencies?

Encouragement. If you have a friend or loved one who has panic attacks, the best thing you can do is to encourage that person and continue to be available to him/her in whatever relationship you currently have. Also, encourage this person to seek out care and to stay consistent with treatment. Continue to show the care and concern that is part of a positive relationship.
Understanding. When anyone suffers from a serious disorder offering them understanding can be quite an important means to show your support. For the panic disordered individual, have them describe what it is like when they experience an attack. Ask them what you can do to help prevent an attack and what can you do should they experience an attack with you present. Let them know you are there for them.