I think it is iffy as to whether there is an actionable defamation claim since it is not only unclear as to whether what was said was in the form of "opinion" or a "false statement of fact" , but because the "investor" stated that he didn't necessarily believe what was said, but just didn't want to get involved in the "drama" - presumably meaning the discord between the two of you. But I question whether under CA law, his actions might not constitute "tortious interference in a business relationship", for which you could seek the lost investment as a measure of damages. You should sit down with a business litigation attorney for a consult on the issue.
If the damages are substantial and truth in fact is on your side, consult a defamation attorney in CA.
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It is not an easy question and serious consideration has to be made wth an aottorney for what damages you seek, and if you can collect on the potential judgment for those damages if you fle a law suit. If the relationship is professional , perhaps a cease and desist letter could prevent further controversy.
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