Totally aside from what "feels" and "seems" good or bad, the only relevant question is whether OP kept their shares or not when they left... this should be the only point they have to prove, no? And what about the patent? Was that bought or licensed from them? I think we are lacking the important details, namely what happened when they left the company and what they signed then and before.

So it comes down to what papers were signed when by who and do you still have copies? If all you have as proof is "word of mouth" without any witnesses then, well, you might be fighting a lost cause.