How to deal with a cancer survivor as a partner?

Your friend is a survivor, but her battle with thyroid cancer does not end, in anyway, with the surgery, with the RAI treatment, or even the (very slow) return to hormone balance... it is good of you to seek some help with understanding what she is going through.

First, thyroid cancer is cellular - so when we have our thyroids removed, it does not normally 'get all' of the cancer cells. Even the very best surgeon in the world must leave tissues behind, to protect the vocal cords, airways, etc.

When all the hormones are removed from our bodies, as they are for our RAI/I-131 scans or treatments, it is a huge impact on the body and mind.

These hormones manage the 'gas' for our bodies; the speed of our heart rate, blood pressure, temperature control, thinking and motion. Physically and psychologically, the hormone imbalance alone can be almost unbearable for some of us - even without the worries of not knowing if our cancer is truly gone.

Once we've had surgery, then our RAI treatment, in 6 months to a year it may be 'gone' - for now. But unlike many other cancers, thyroid cancer requires dilligent follow up, for the rest of our lives; constant balancing and rebalancing of the hormones, to prevent any tissues from regrowing, and/or providing 'food' for any remaining cancer cells. So it isn't over for us... ever. That can be a difficult thing to wrap our minds around, too.

Depression is a 'normal' side effect of hypothyroidism, too - and we are normally required to be 'hypo' for all early scans and/or treatment dose of radioactive iodine, to make it work better.

Once our treatment is done, it takes 6 weeks to 6 months to get our hormones into 'suppression' - which is, technically, hyperthyroidism. We have to do this to prevent the tissues from regrowing, but it can be very, very difficult, too - as you can read in this link, a very normal side effect is anxiety, sometimes severe, as well as depression - search for 'thyroid' in this paper, for study references:
http://www.drrichardhall.com/anxiety.htm

This briefly explains some of the physiological things that thyroid hormones impact:
http://arbl.cvmbs.colostate.edu/hbooks/pathphys/endocrine/thyroid/physio.html

And page 4 of this shows the 'normal' side effects of hypothyroidism, which can take a long time to recover from after we are hypo for treatments or scans:
http://www.thyroid.org/patients/brochures/Hypothyroidism _web_booklet.pdf

This is a terrible metabolic disruption for our bodies, as you can see from the above.

Severe hypothyroid states can even bring on psychosis, and a life threatening crisis, particularly if there are any other health issues (ie infection, etc) at the same time:
http://www.emedicine.com/emerg/topic280.htm
http://www.indegene.com/End/FeatArt/indEndFeatArt7.html
http://www.psychiatrist.com/pcc/pccpdf/v05n06/v05n0603.pdf

The best thing you can do to help is to understand that this is not just having a hard time dealing with the cancer (and we do not know if it has been fully removed, that is not the way this works...); it is often a response to the body being deprived of absolutely essential 'gas' in the thyroid hormones.

I hope this helps a little. I know that it took my husband a long period of time to learn about the functions of thyroid hormones, too - and it was a rough ride. One minute I could be laughing, and the next I was literally bumping into walls, in tears, or in a rage - and I couldn't even begin to understand why I felt so badly, so it was almost impossible to explain what was going on. If you learn all you can about what she is going through, and about the critical importance of thyroid hormones to our entire body, it will help her - if it is anything like I went through, she is not herself at the moment, and truly cannot help it. All the best to you, and I hope things work out ok. There is a Thyroid Cancer friends and families/caregivers group on Yahoo, too - you may want to try that, to talk with others who have been through this as a loved one or friend of a thyca patient:
http://www.thyca.org/email.htm#caregivers