If my partner abandons our business does he forfeit his ownership

This answer should not be considered specific legal advice, nor does it establish an attorney client relationship. Generally, this answer may depend on what your partnership agreement provides, but barring specific provisions in the agreement, a departing partner would normally be entitled to the value of his interest in the business. However, a failure to fulfill responsibilities as a partner can give you a defense or a set-off. This can be complicated to unravel, so you really should talk to an attorney on this if the business has any value.

Your question is very bare bones but depends upon quite a few factors. Is is an LLC or a corporation? Is their an Operating Agreement or Shareholder Agreement in place? Is your partner looking to resign from the operations but still have responsibility for capital contributions and liabilities? These are all questions that need to be answered. You probably want to meet with a lawyer to discuss these issues.