He has a lot to lose:
* The 3% he's been offered, which while paltry compared to the 40% he had if things had gone better is still not a small amount of equity to hold in a successful company.
* Many thousands of dollars in legal fees.
* Many months of effort and stress.
People on message boards have weird ideas of what does and doesn't constitute lawful termination. Hiring in the US (I'm assuming this is the US, because there was a $100k investor) is at-will, most especially so in companies documented carefully enough to have 4/1 vesting. There is likely no such thing as a "bad faith" termination; there is only termination authorized by contracts establishing who manages the company, and termination that isn't.
Again, by all means, pay a small amount of money to get a competent lawyer to verify that's the case. Everybody on this thread will likely say "go ahead and fight" if it turns out this person can't be fired, so you can just stick a pin in that thought and we can continue to discuss the more realistic scenario.
Threatening to sue: also often a bad idea! If it's not credible, and everyone knows you don't have the resources to litigate, and that even if you did it would be economically irrational to do so, then the threat does the opposite of gain you leverage; meanwhile, threats can trigger other legal problems for you. Advice I'm pretty confident in giving: don't trust a message board post that tells you to threaten legal action.