Vertigo can be caused by inner ear disorders and can keep you off your feet for quite a while. My daughter-in-law went on a cruise to Alaska and it took a couple of months for her to get over the vertigo she experienced when she disembarked. Have you experienced vertigo?
Not really sure if it was vertigo but after a cruise it will take me a few days to get my land legs back. I can feel the movement of the ship especially when I am sitting still or laying down. But that is usually after a cruise where there was a bit more rocking and rolling that usual. I have been on about 10 cruises and it has only happened a couple of times and usually on 7+ day cruises. Never after traveling on a plane, though.
I've had ear problems my entire life. I get vertigo after almost every flight and cruise I take. It usually subsides after a day or two but in some cases, it's taken weeks and months. I take Zofran for the nausea. An acupuncturist might help your daughter-in-law. I went to mine afterwards and she was able to greatly alleviate my discomfort. If this is the first time she's had this, she may want to visit a doctor to see if there is something more serious causing it. My ears are exceptionally messed up so there is no surprise with me. It is a very difficult sensation to deal with and people who have never experienced it don't understand how bad it can be. Also drink a lot of water and check blood pressure. If it's low, I think it can potentially exacerbate the issue since dizziness is already prevalent. But I'm no doctor! Good luck to her!
Cruise ship made me so wonky!!