Hello,
The Monetary widget can be used in list views as well, but the "currency_id" field must be present within the tree itself as well.
For example, my invoice form did not have the currency for the price_unit field, so I was able to add it with the following custom view:
<record id="invoice_pretty_price_form_tree" model="ir.ui.view">
<field name="name">Account Invoice Pretty Price Form Tree</field>
<field name="model">account.invoice</field>
<field name="inherit_id" ref="account.invoice_form"/>
<field name="arch" type="xml">
<xpath expr="//field[@name='invoice_line_ids']/tree/field[@name='price_unit']" position="before">
<field name="currency_id"/>
</xpath>
<xpath expr="//field[@name='invoice_line_ids']/tree/field[@name='price_unit']" position="replace">
<field name="price_unit" widget="monetary" options="{'currency_field': 'currency_id'}"/>
</xpath>
</field>
</record>