1. Define an attribute attrs on some form element:

<field name="parent_id"
       attrs="{
           'invisible': [('is_company', '=', True)],
           'required': [('is_company', '=', False)]
       }" /> 

2. Take care that all the fields you refer to are available on your form:

<field name="is_company" invisible="True" /> 

This will make the parent_id field invisible if the partner is a company, and required if it's not a company.