If 'Alfie' was in B1, you can INDEX the column you need via something like
=INDEX(emp_table, , MATCH($B$1, emp_table[#Headers], 0))
Plugging that into your original formula
=sumifs(INDEX(emp_table, , MATCH($B$1, emp_table[#Headers], 0)),emp_table[[Weeknum]:[Weeknum]],$A2)