Thank you for trusting JustAnswer with your important question.
If you suspect Huntington's, then you should mention something to her and her spouse. Medical reasons (including thyroid issues, diabetes, hormones, etc) should always be ruled out in cases of significant depression or personality changes like you describe. And given the severity of what you describe it's possible that she has both a medical and a psychological issue going on at the same time.
You need to approach her gently, from a caring and supportive place, not a critical place. Let her know you're worried about her, and you want to see her be healthy (especially if she's trying to have a baby too).
What I don't hear that we'd expect in Huntington's would be abnormal movements/tics and restlessness. But she could still have other medical issues causing or worsening the depression.