Depression and Anxiety

She might have had a minor stroke. Get her to see another doctor… get this ruled out. Personality changes are a symptom of stroke.
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So, …she is never the total opposite from being totally depressed to totally happy? Then why was she diagnosed with BP? Tell her to go to a different doctor.
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Depression is VERY common when one is having multiple health problems. (Situational depression) Bi-polar disorder USUALLY comes around age 30 - 40 tops. She needs a full work-up by someone other than a General Practitioner who's going to throw meds at the wall and see what sticks. A Psychiatrist who employs therapy is probably a good bet, although a thorough physical should be the first step. Preferably at a major medical center.
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You can get linked up with NAMI,
http://www.nami.org/

A psychiatrist goes to school for 12 years. Bipolar tend to show up in the late teens. Lithium is standard. It can take up to two years to get the drug balance right. In the meantime, follow up and go to a support group that deals with friends and family with mental health problems. You are not alone.
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what you are presenting is not a bipolar. depression comes into existance in these ages mostly because of age dependent alterations in neurotransmitters and also because of other health problems as you said. if she is insured you need to change her doctor and discontinue lithium. be aware of kidney related problems of lithium and check the blood level of lithium.
a question for you just popped up in my mind: are you sure that your friend has had no history of manic depressive episodes in long past? if you are in two minds, it will be double checked.
good luck
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I am a woman age 60 & dr.s had labeled me "Cyclothymic" which is a mild form of Bipolar. BUT the criteria for true Bipolar is to have episodes of days or weeks of mania where one does not sleep & has grandiose ideas of doing all kinds of things. Depression can be just that, if she is taking Lithium for just depression that seems like too hard of a drug for that. I take Prozac for depression & the only "high" I get from time to time is I will get obsessed over something & can't stop (like a hobby) But I do sleep & it is very mild compared to a full blown Bipolar illness. I honestly would think by the age of 61 she would of had some signs of bipolar long before now. My guess is that is could be a side effect or reaction to some medications & it could be drug induced symptoms that look like Bipolar.
Has she had her thyroid checked ?
I think she should go to a differant dr. ASAP.
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if she has never had a manic episode she cannot have bipolar I disorder (by definition). lithium is a serious medication and should be taken only if necessary. if the current psychiatrist is not willing or able to explain why he/she arrived at her current diagnosis and why he/she has chosen the medication, it would be worth getting a second opinion. i would definitely try to discuss it with this doctor (maybe your friend would sign a release of information so that you can talk to the psychiatrist directly) before deciding to change; he/she may have more information to shed light on things. good luck!
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i'm a psychiatrist

I am bipolar 1, which wasn't diagnosed till my late 40s. Of course, I was an alcoholic until I was 39, and am told that's a form of self medication and the disorder probably started in my teens. It's rare for BP to show up later in life, so I'm told. You need to get your friend to a psychiatrist for evaluation. Don't depend on an MD. It seem lately, anybody who turns up with a headache is treated as being bipolar. It's the going diagnosis. I've also been told that if there is no history of manic episodes, the condition doesn't qualify as BP. Be careful, though, a manic episode can occur and someone simply think they're have a really good few days. The lithium won't hurt your friend, unless she's being given a very heavy dose. It is somewhat toxic, and can damage the liver and kidneys, but it takes a long while. Go see a shrink for a thorough evaluation. If nothing else he can put her on the proper anti-depressant to deal with her depression (they are not all the same).
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I agree with everyone else that has stated to GET ANOTHER OPINION.

If the Lithium isn't helping, it probably is not the problem. Lithium works incredibly well with Bi-polar disorder.

Is this a family doctor (I assume) or a psychiatrist? See if your friend can be referred to a psychiatrist instead. They have FAR more knowledge about disorders and may see something that a family doctor is overlooking (sometimes due to a simple lack of time with their patients).
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Plenty of good advice here.
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http://www.bipolar-disorder.jims-info.com/