pos/pos_customer_required/views/pos_order_view.xml

<?xml version="1.0"?>
<!--
    Copyright (C) 2004-Today Apertoso NV (<http://www.apertoso.be>)
    Copyright (C) 2016-Today La Louve (<http://www.lalouve.net/>)

    @author: Jos DE GRAEVE (<Jos.DeGraeve@apertoso.be>)
    @author: Sylvain LE GAL (https://twitter.com/legalsylvain)

    The licence is in the file __openerp__.py
-->

<odoo>

    <record model="ir.ui.view" id="view_pos_order_form">
        <field name="model">pos.order</field>
        <field name="inherit_id" ref="point_of_sale.view_pos_pos_form"/>
        <field name="arch" type="xml">
            <xpath expr="//sheet" position="inside">
                <field name="require_customer" invisible="1"/>
            </xpath>
            <xpath expr="//field[@name='partner_id']" position="attributes">
                <attribute name="attrs">{'readonly': [('state','=','invoiced')], 'required': [('require_customer','=',True)]}</attribute>
            </xpath>
        </field>
    </record>

</odoo>