Add foreign accepted mail domain, Spoofing

Hi, 

Because of not knowing what how its is called official the subject description is a little bit weird.

We wish to mail in name of a business partner of us who does have his own O365 tenant.

Mail from this partner should be forward to us and our callcenter agents must answer with an email address of there domain. 

Normally this is done with SPF, DKIM, DMARC configuration.

But how can I accomplish this in a environment with two separated  Office365 tenant.

I know this is a little strange (actually a stupid) question in fully cloud situation. Why not create an account in the business partner tenant let agent login over there and done.

But what the hack the bosses demand a solution where are able to send mail in name of a domain not owned by us.

Kind regards.