Using integrated security over VPN We've got a client that we're going to allow to use one of our applications.  Staff on site regularly connect to our network via VPN.  The .Net application that connects to the SQL Server uses integrated security.  The laptops they use are members of our domain.

The client has requested use of the application as well.  So we gave them a network account and the ability to create a VPN session on our network.  I gave that account a login on the SQL server with the same rights as our own staff.

The problem is that when they try to use the application they get the following error:  The login is from an untrusted domain and cannot be used with Windows authentication.  

I'm guessing that since they are using a computer that is not on our domain, it doesn't matter that they are using a login with the VPN that is.  So, what is the best way to address this problem?  I'd rather not switch over to SQL authentication for this small application.
If the client computer isn't on the same domain as the SQL Server, and if there's no trusts set up between the client computer's domain and the SQL Server's domain, you can't use integrated authentication.