Please read carefully the docs before posting some issue – here is what should be read
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“Hides a column with a given colname. If the colname is a string, only the specified column is hidden. If the colname is array of type [“name1″,”name2″] then the columns with names 'name1' and 'name2' will be hidden at the same time. The names in colname must be valid names from the colModel. The width of the grid is changed according to the following rules: if the grid currently has no horizontal scroll bar, the width of the grid is decreased by the width of the hidden column(s). If a scrollbar is visible, the width is adjusted which may or may not change the width of the grid.”
This should work, but I sugest to cal this not in afterInsertRow, since this is executed every time you add row. If possible please use gridComplete event for this purpose.