pos/pos_customer_required/views/pos_view.xml

<?xml version="1.0"?>
<openerp>
    <data>

        <record model="ir.ui.view" id="view_pos_config_form">
            <field name="name">pos.config.form.view</field>
            <field name="model">pos.config</field>
            <field name="inherit_id"
                   ref="point_of_sale.view_pos_config_form" />
            <field name="arch" type="xml">
                <xpath expr="//group[@string='Receipt']" position="after">
                    <group string="Customer" col="4" >
                        <field name='require_customer' />
                    </group>
                </xpath>
            </field>
        </record>

        <record model="ir.ui.view" id="view_pos_pos_form_inherit">
            <field name="name">pos.order</field>
            <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>

    </data>
</openerp>