CRM for outlook 2015 question

I have a CRM environment in my network we have a partner company(totally different domain) that's accessing the environment via VPN  they have AD accounts in our domain....

What I want to achieve is the following....

1. Have the users from the partner company access CRM without having to have an AD account in our domain (using their own AD credentials)

2. Get the outlook client to working because currently they are logged in with their own credentials (they use our AD just for the VPN connection however on their PC they are logged in with their domain) and the outlook does not work.

 Is there any way to achieve what I want maybe via ADFS?

Any suggestions are welcome,

Thank you in advance