Hello, As you have not mentioned your mothers age in this post I m considering her to be somewhere around late 40 s. As an adult, she can tolerate upto 500 to 600 mg of acetaminophen given no history of liver disorders. So you need not necessarily worry about the interaction. Moving on, if she is just having a day of cold symptoms without fever you can of course try Nyquil along with Robitussen for the cough only at night. Additionally, she can also try steam inhalation and humidifiers for symptomatic relief. Ask her to remain well hydrated. If there is nasal congestion she might try nasal saline sprays as well. These are all supportive measures and have no adverse effects. If she develops fever, abdominal symptoms or respiratory difficulty then it is advisable for you to visit you PCP, just to rule out flu or strep. At this point of time assuming it to be just a day of symptoms I would say this is likely viral and a post viral cough syndrome.