partner-contact/base_partner_sequence/views/partner_view.xml

<?xml version="1.0" encoding="UTF-8" ?>
<odoo>
    <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>
</odoo>