However, since OPs cofounder paid out of pocket for the app, I'm not sure what the StartupInc actually owns. But that also means the possibility of piercing the corporate veil - which is also bad.
I'm not a lawyer, but I did have a startup where my cofounder gutted the company after trying to stage a coup - then created a very similar company. He'd signed all the NDAs, non-competes (these ARE enforceable, even in CA, with co-founders/execs) etc.
I never went after him.
Instead, I waited to see: would he be able to steal my idea, connections, and clients to be successful on his own. If so, I planned on getting my share through legal proceedings.
Short version, a few years later one of his 3 co-founders quit, a year later the other did, and now the company is gone.
I'll avoid giving OP advice based on my one story, but another poster made a good point: there is very little to be gained.
Legally, though, I believe that - short of creating a new corporate entity or watering down the stock (both possible, though they come with risks RE getting sued by OP) - OP will retain all of their shares.
Of course, being not a lawyer, there are likely major gaps in my understanding and OP should at least have a chat with one.