Hi,
its still best to approach with communication first. The idea is to contact them and let them know you know… that you’re not going to stand by and do nothing… and then if they don’t comply … use stronger tactics.
I think it was someone thats a member here whom I recall had hired someone else for SEO purposes; and the SEO people copied content and added it to the person’s website without them knowing what had been done.
once the person was aware, they took down the content and that was that.
While that may not be the case here; its certainly worth the time it takes to contact them…. which it seems you’ve done. Now i’d give them a day or two to respond before going any further.