1. Our client is using a European Office 365 Tenant. 2. The client wants to implement Yammer. 3. The client was told by a 3rd party that the underlying Active Directory data (e.g. name, e-mail address) is replicated to the US. (As Yammer is currently only hosted in the US by Microsoft). 4. The client does not want to have his AD data replicated to US-based servers
Just in case you haven't seen it, the Where is my data? site is useful for these sorts of things, though I don't think it will tell you anything you don't already know.
It would be good to get an update on local data residency option especially with GDPR impending and any implications this might have. This was that last public update I have seen from here, which says this option is in preview currently (for new tenants only) but GA not expected until Q2 next year when a general rollout to existing tenants will start.