Unless there has been a WRITTEN transfer of rights in your work to your partner, you own the rights to your work and your partner has no right to use it.
Under the Digital Millennium Copyright Act, service providers can be held liable for copyright violations by their clients unless they take prompt action ("prompt" is not defined in the law) to remove or block access to material published by their clients in violation of copyright. This aspect of the law is referred to as "Safe Harbor". Under the Safe Harbor provision, ISPs must have a designated agent who handles reports of copyright violation.
I'm not going to take a lot of time to describe the law; you can easily Google "DMCA Safe Harbor" to learn more. I've blogged about the procedure for filing a copyright complaint here:
Real Estate Blog - What to Do When Your Blog or Graphics Are Stolen