Per their website:
Frontier Airlines acknowledges that some of our passengers suffer from mild to severe peanut allergies. For this reason, we do not want to create false expectations with regard to the aircraft environment. We are unable to guarantee a peanut or allergen-free flight, nor can we prevent other customers from bringing peanuts or products containing peanuts onboard our flights. Some inflight food offerings may contain peanuts, peanut butter or trace amounts of nut ingredients, or may have been processed in facilities that handle nuts, including peanuts. Prior to making travel plans with Frontier, we urge you to speak with your health professional regarding risks of onboard exposure to any allergen.
Frontier does allow passengers to travel with pets in the cabin of our aircraft. These pets may include domesticated dogs, cats, rabbits, guinea pigs, hamsters, or small household birds. Therefore, we recommend that passengers with allergies to pets check with your health professional regarding risks of onboard exposure to any allergen. For more information on travelling with pets, visit our Traveling with Pets section.