Not a whole bunch you can do about that, if you flat-out don’t know what domain they’re in at the time. You need to be able to tell the command what domain to query.
The manager field output should be a distinguished name, so you could do some string manipulation to get the domain from there. For ease of use I’d write a function called get-domainfromDn. Then your select scriptblock could be
{get-aduser -properties mail nickname -identity $.manager - server $(get-domainfromDn $.manager) }
Assuming your credentials are good for the other domain it should work.
Easier solution and I hate to do it because I prefer to stay native, but I have other work to do! The Quest snap-in recognizes the domain of the object for which the property you are trying to retrieve.