pass through credentials for sharepoint list on ipad

I have a SharePoint Calendar that users are viewing on their IPAD, and I have the link to the Calendar saved as an icon on their home screen.  
 But everytime they open it they have to login with their username and password on the MS Forefront TMG login screen, to get to the SharePoint Calendar.   This is annoying for the users.

Is there a way to pass through their login credentials to this SharePoint Calendar, so they can by-pass logging in everytime?

Their accounts are domain accounts in Active Directory.

Please help.

thank you!