To prepare a grid which is the result of a JOIN or a complex query, is there a way to create the pop-up editor interface to edit both of the related records via interacting with that single row?
If your refer joins as the lookup tables from the main table, it is already built with phpGrid. Use "select" edit type in set_col_edittype() function.
More about set_col_edittype() https://phpgrid.com/documentation/set_col_edittype/
More complex relationships can usually be handled with a master detail or subgrid in the commercial version.
Other Recommended KB: https://phpgrid.uservoice.com/knowledgebase/articles/67197-complex-sql-query