This simply can't continue.
OK... .
Is there anything unusual about this client? Anything that would be of a personal nature or set them apart from other clients.
Any reason for your partner to "view" this client as something other that business?
OK... let's confirm.
In the presence of another person (this client) your business partner said.
1. that he wasn't feeling well
2. that you "stole everything" from him (were there any specifics?)
3. and then vaguely insinuated that he wouldn't be around or didn't know.
furthermore
He told this client that YOU had made promises that you wouldn't fulfill... .
Is there any part of that I have wrong?
I'm not an expert on libel/slander, my gut says he "skirted it" on this occasion, but perhaps the laws of your state are different.
You are in a business setting, trying to keep or get more business and someone tells (without any basis in fact), the person whose business you are trying to win, that you are untrustworthy (stealing and not keeping promises).
Perhaps it's not skirting it.
YOU need to talk to a lawyer ASAP. I would guess the lawyer would send a "cease and desist" letter immediately. No more talking to anyone about you stealing or you not following promises.
You need to protect your business and your professional reputation right now... .right now.
I'm so sorry it has come to this.
FF