N.Y. Gen. Oblig. Law § 15-105

§ 15-105. Discharge of one obligor, without reservation. 1. If an\nobligee releasing or discharging an obligor without express reservation\nof rights against a co-obligor, then knows or has reason to know that\nthe obligor released or discharged did not pay so much of the claim as\nhe was bound by his contract or relation with that co-obligor to pay,\nthe obligee's claim against that co-obligor shall be satisfied to the\namount which the obligee knew or had reason to know that the released or\ndischarged obligor was bound to such co-obligor to pay.\n 2. If an obligee so releasing or discharging an obligor has not then\nsuch knowledge or reason to know, the obligee's claim against the\nco-obligor shall be satisfied to the extent of the lesser of two\namounts, namely (a) the amount of the fractional share of the obligor\nreleased or discharged, or (b) the amount that such obligor was bound by\nhis contract or relation with the co-obligor to pay.\n