ISA server- Bypass authentication

Hi 

My environment: External users access SharePoint intranet site by entering credentials in Microsoft ISA server login page(authenticate to ISA server then accessing all sharepoint sites).

one client wants to access sharepoint intranet without ISA authentication.Is there any way to access SharePoint intranet site(https://domainname/sites/site1) from internet without ISA authentication.I mean bypass ISA proxy authentication for this particular SharePoint site(https://domainname/sites/site1)

SharePoint site(https://domainname/site/site1) is enabled with anonymous authentication.

Thanks for any help.