Anyone who had at least one brain cell would know better than to believe anything that came out of her mouth when she was referring to her condition. Because more often than not, she would make light of whatever she was going through. However, an individual with more than one brain cell such as Snape also knew better than to convince her to do otherwise. She was infuriatingly bullheaded, just as her father had been. She was a true Potter, that was undeniable. So, Snape simply accepted her fake vow and let her go, wandering off to a different direction to reunite with his fellow Death Eaters.