Visit your local Police station and let them know the score. To deprive your partner of his tools of trade is a criminal offence of theft, if she disputes the bill she should dispute that through a Solicitor using civil law procedures not by taking the law into her own hands & illegally detaining his tools.
Ask the Police to provide a escort for him to her premises whilst he seeks the return of his equipment, failing that she could be charged with theft.
Could you imagine what it would be like, if garages refused to return customers cars everytime the garage bill was disputed by the customer.?