I think a lawyer could effectively argue for your case, and it is possible you could win. It would be a litigation issue, though, which is expensive no matter who wins. Please see an IP attorney in Cali to advise you in the matter.

"Contract" is one of the branches of law when the classification is based on the relationship between the parties, and other branches are "torts" and "crimes." Contract cases are usually based on voluntarily-assumed obligations between the parties. "Intellectual property" is part of a classification based on type of property, and others would be real estate and personal property. These classification systems cross over, i.e., there can be crimes involving intellectual property and contracts involving real estate.

I'd suggest thinking about your situation as perhaps involving partnership law, which I suppose is a subspecies of contract law. Your business relationship seems to meet the test of being a partnership, and the test for the existence of a partnership does not require either an express agreement (written or oral) nor an intention to form a partnership; it only requires that there be two or more persons, working together in a for-profit business with the intention of sharing the profits. There's a bit more to it, but basically if your relationship was of this nature, partnership law probably applies.

If you have or had a partnership, the California version of the Revised Uniform Partnership Act (RUPA) would, in the absence of an express partnership agreement, govern your respective rights, obligations and duties. The RUPA contains provisions for contributions to partnership capital, definitions of dissociation and withdrawal by a partner, whether a withdrawal is rightful or wrongful, rights and duties of the remaining partner under both rightful and wrongful withdrawal scenarios, buyout provisions, and more.

The law is too complex to quote or apply to your facts here. Nevertheless, it is fairly straightforward, and it may be helpful for you to obtain and read (or at least skim) the RUPA. It is found in (of all places!) the Corporations Code, as sections 16100 to 16917.

There is no way to give you a satisfactory answer to your post because there are missing facts. The information you provided leads me to believe you may fall under California partnership law that provides for automatic provisions when two people enter into a joint venture without a written agreement and acted in reliance thereof. (The law does assume 50-50 unless otherwise provided.) There are other issues and additional facts could color this initial impression or change the nature of the remedy you would be seeking. Next steps are critical and depend on the current state of the relationship with your friend. If at all possible, it would be preferable to negotiate an outcome. You definitely need the advise of good counsel if this potential product is ever going to be marketed.