Cannot change cart email within Craft Commerce while logged-in
I don't think there is an easy workaround here as you're dealing with logged in user functionality, and by definition that's not really designed to be easy to workaround.  

Probably the easiest thing is for the admin to use the 'log in as XXX' functionality, and then complete the order for the customer (assuming the customer had a Craft account which is not, per se, a given with Commerce).  If they're guest customers, the admin should probably jsut log out, complete the order as a guest, and then log back in, basically.

Order entry for admins (via the backend) is on the Commerce plans, though, so a better solution may well appear.