Are you also a co-founder or just an employee that didn't get taken along. Only matters in terms of standing nothing else, e.g. an employee wouldn't have stand

Are you also a co-founder or just an employee that didn't get taken along. Only matters in terms of standing nothing else, e.g. an employee wouldn't have standing to go after a co-founder for doing this AFAIK, but a fellow co-founder or investor could likely. If you are an employee with vested shares, then technically you might have grounds to go after him for causing a devaluation of your stock. But that only counts if it actually has/had real value at the time he left. In the end, all this depends, and IANAL but if the IP was developed before the company was legally formed and never properly assigned to the company then it might be a little tougher to go after him if he originally created it. At the same time, if the company paid to develop features etc and was formed legally, then there could be a claim, again it is likely messy. You can absolutely find some good attorneys to at least sit with you in consultation and figure out where you stand. Honestly, even if it costs you 2-4 hours of their time that would be worth it.