It doesn't matter if a legal contract was signed. As long as you undertook this venture with the agreement (verbal) that each person owns a part of the business

It doesn't matter if a legal contract was signed. As long as you undertook this venture with the agreement (verbal) that each person owns a part of the business, than it's the same as forming a legal partnership. Though, it wouldn't be useful to sue her unless you guys specifically agreed that she would finish the work, etc...