When this event fires, the contents of the editor have not been applied to the grid. You can validate the editor contents and cancel the edits if necessary.

To validate the editor contents, check the value contained in the Editor.Text property. If the value is invalid for the cell, set the Cancel parameter to true and the grid will remain in edit mode until the user types a valid entry.

void _flex_ValidateEdit(object sender, ValidateEditEventArgs e)
{
    if (_flex.Cols[e.Col].Name = "Score")
    {
        try
        {
            int value = int.Parse(_flex.Editor.Text);
            if (value >= 0 && value <= 50)
                return; // accept edits
        }
        catch {}
    // error or invalid range, refuse edits
    e.Cancel = true;
}

}