If field_id is a many2one field to partner, then you can simply give domain in xml for product_id field. Domain should be domain="[('field_id', '=', parent.partner_id)]"

Janeesh - here is my attempt to implement your solution in a module. The domain isn't showing up for me right now, and I think it is because of my xpath declaration. Could someone help me figure out how to declare this domain correctly? Thanks!

EDIT: For anyone wondering the solution here, I think it had to do with the fact that the 'field' tag I was looking for was embedded past another field tag. The xpath that ended up working for me was:

 <xpath expr="//field[@name='order_line']/tree/field[@name='product_id']"

which actually could be shortened to 

xpath expr="//tree/field[@name='product_id']"

Here is my original (not functional) code.

   

    <record id="view_quotation_tree" model="ir.ui.view">

        <field name="name">sale.order.form</field>

        <field name="model">sale.order</field>

        <field name="inherit_id" ref="sale.view_order_form"/>

        <field name="arch" type="xml">

           <xpath expr="//field[@name='product_id']" position="replace">

              <field name="product_id" context="{'partner_id':parent.partner_id, 'quantity':product_uom_qty, 'pricelist':parent.pricelist_id, 'shop':parent.shop_id, 'uom':product_uom}" groups="base.group_user" on_change="product_id_change(parent.pricelist_id, product_id, product_uom_qty, product_uom, product_uos_qty, product_uos, name, parent.partner_id, False, True, parent.date_order, False, parent.fiscal_position, False, context)" domain="[('x_cust_name', '=', parent.partner_id)]"/>

           </xpath>

        </field>

     </record>