Try inheriting the view in a new xml file:

<record id="res_partner_x" model="ir.ui.view">
        <field name="name">res.partner.form</field>
         <field name="model">res.partner</field>
         <field name="inherit_id" ref="base.view_partner_form"></field>
         <field name="arch" type="xml"> 
                <xpath expr="//field[@name='function']"" position="replace">
                       <field name="function" placeholder="e.g. Sales Director"
                       attrs="{'invisible': [('is_company','=', True)]}" 
                       class="oe_edit_only"/>
                </xpath>
         </field>
</record>

This way, the field will also be invisible while in edit-mode.

Another suggestion would be to make the field readonly.

<field name="function" placeholder="e.g. Sales Director"
       attrs="{'invisible': [('is_company','=', True)]}" 
       readonly="1"/>
</field>

This way the field will be visible but not editable.