Hi redfoot1, welcome to WhatNext. I am so very sorry for the reason that you're here - your wife is so young to have been diagnosed with lung cancer!!! But, I'm glad you're here and I'm glad that you've already jumped in with a question. I wish I knew the answer to it - I was treated with Opdivo, a form of immunotherapy for 5 years, but have not experienced Tagrisso. On Opdivo, I had no issues except a thyroid that quit working correctly. Is she only on Tagrisso? What does her oncologist have to say about her nausea? I don't know for sure, but that seems like a relatively rare side effect for immunotherapy. Perhaps they could try either Keytruda or Opdivo and see if she tolerates one of those better.
I would like to introduce you to Lungevity.com as well. This is an organization that is packed with people with lung cancer, including young people. The pandemic is throwing wrenches into their normal operations, but they are very supportive of lung cancer survivors (which we all are from the time we are diagnosed). There is also a specific group called EGFR-Resisters:
https://www.facebook.com/groups/EGFRResisters/ There are over 2000 people on this FB page, I presume all of whom have been diagnosed with the EGFR mutation.
Good luck. This is a very supportive and caring site so I hope to see more and more of you here as you go through this experience with your wife. Caregiving is difficult and we are here for you.