partner-contact/base_partner_sequence/partner_view.xml

<?xml version="1.0" encoding="UTF-8"?>
<openerp>
    <data>

        <record id="view_partner_form" model="ir.ui.view">
            <field name="name">Make partner reference readonly when non-commercial</field>
            <field name="model">res.partner</field>
            <field name="inherit_id" ref="base.view_partner_form"/>
            <field name="arch" type="xml">
                <field name="ref" position="attributes">
                    <attribute name="attrs">{
                    'readonly': [('is_company', '=', False),
                                 ('parent_id', '!=', False)]}</attribute>
                </field>
            </field>

        </record>
    </data>
</openerp>