I definitely think it's possible to go from hypomanic/manic to depressed in one day. I think with bipolar disorder, switches of all sorts can be possible. I know that for me, I've had similar super quick shifts, though they were usually from moderate to moderate or mild to mild and not severe to severe. Usually when they have been very rapid and fluctuating there has been an antidepressant involved. Or, I often transition via mixed episode, though I have also had long stretches of stability in between. Sometimes I even go from hypomanic or manic to stable and then back to hypomanic/stable again (or vice versa with depression).

I really like Whatever2013's, quote by Dr. Seuss.

If your friend does have bipolar disorder, or some other significant mental health issue, I hope she will soon gain more insight to it and acceptance. It's not healthy to deny, for many reasons.