There's a whole bunch of different ways you can go with this, and almost all of them are feasible if not elegant. Sharing exchange calendaring fully between 2 unconnected AD domains is possible with external tools like INTERORG, or companies like Quest have some GAL-Sync tools that may be useful.
I would look at either:
Hosting their mailboxes on your server, pointing their external domain MX to your server, and serving their mail to them over OWA or RPC. Or...
Migrate the users into your AD, switch out their current DC for a DC in your domain, and serve their mailboxes on an exchange server (in your organisation) hosted in their office.
There really isn't a single 'best way' to achieve this, it depends on a bunch of factors like the connection quality between you and them, how integrated into your org they are (in non-IT terms) and how vital their computer infrastructure is to the operation of their day-to-day business.
If you have a rock solid WAN connection to the office, and there's little chance that they're going to be sold off from the business in the foreseeable future, I'd lean towards the 'extend our domain to their office and decommission their existing domain' approach.
On the other hand, if the WAN link is flaky and the business unit might be sold off in 6 months.... keep it as it is. You'll just be migrating everything back when it happens, as the new holder will want all that email history. Put a remote backup in place. Maybe nominate a secretary at their office that has OWA access to your calendars.
This link might be of interest to you.