Legal Question in DUI Law in Washington

No it does not. You have two issues here: the use and possession of marijuana by a person under 21 years of age in Washington State and driving with THC in her bloodstream. Your daughter is an adult in the eyes of the law but she may not possess marijuana because she is under 21 years of age. The driving with THC in her system could subject her to a minor (under age 21) drug DUI if she has any THC in her blood at all. THC will stay in a persons blood and body and is detectable for up to thirty days with the modern testing procedures for urine, blood and hair follicle testing. So smoking one day and then not smoking the next day when a person drives does not solve the problem of THC in the blood. She should speak with her medical practitioner to see if taking CBD products may help her with her anorexia and help stimulate her appetite or by using some other therapy or prescription drug or herbal or homeopathic remedy. By regularly smoking marijuana and driving she is exposing herself to being charged with a minor DUI/Drugs. Then there is also the point you make about her being involved in an accident and the police testing her blood for THC afterwards, even if the accident is not her fault.