I would create a view on the server that your customer logs into.
Example.
select * from [local table name]
union all
select * from [remote server].[remote db name].[schema].
This will allow you the flexability to specify columns in case the table structures are different. You can also perform joins like this.
select a.col1, a.col2, b.col3
from table1 a
inner join [remote server].[remote db name].[schema].
b
on a.col = b.col