Her tantrums, yelling and negative behavior is affecting all of us. What should we do?

Our kids tell us a lot more through their actions about their feelings and what they're experiencing than through their words. In this specific example, we're talking about a seven-year-old who is sweet and kind to the core but sometimes wakes up angry and out of control emotionally. As her parent described, they never know which version of their daughter will wake up - the angel version or the devil version. All of our kids have a tendency to do this. And as the adults in charge of their care, it is our job to figure out what may be leading to the tantrums and agitation.
This is not a simple fix because our young kids (elementary-aged students and preteens especially) don't have the ability to regulate all of their emotions yet. Some tantrums are just part of the equation at this stage - at every stage, really. Adults just typically regulate them better. But there are certainly things that influence tantrums in a negative way. It is important to eliminate those things right out of the gate. Sleep is where I always start. It is a huge factor in behavior, attitude and general health. There are so many influences that get in the way of a healthy amount of sleep so start there and just make sure this isn't an obvious answer. Do you have a consistent sleep schedule for your child? Are they getting enough sleep? Are they waking frequently in the night? Is their sleep environment free from technology (no screens late at night, no falling asleep watching TV, etc.)?
It becomes an elimination game to some degree, eliminating the major causes for acting out. After some of the basic health indicators (getting enough sleep and nutrition, etc.), I would narrow in on some of the psychosocial indicators. Have there been any major changes in the child's life (death of a loved one, physical move, parental separation or divorce, etc.)? Is the child being bullied or having any social issues among friends? Is there anything major that would emotionally disrupt the child's ability to handle or face daily activities? Are their daily routines inconsistent? If any of those are true for your situation, start with that. There are tons of resources out there on how to navigate very specific changes (death, divorce, etc.) but generally speaking validating their feelings and keeping communication open are both very important.
After that, I would do a quick check on the connection between the child and adult(s) in the home. Kids are wired to connect with their adult caregivers. They also soak up our adult feelings and are extremely perceptive. They sense when we aren't feeling well, when we're tired or sad, and they internalize those feelings, wondering if they've caused or contributed to them. Things to consider: How is my attitude around my child(ren)? Do they sense I am experiencing a soul-crushing time in my life? Am I distracted and unable to focus on them when they're in my presence? Am I parenting on my own? If so, is that a change? Are they competing for my attention (with my phone or my job or my new baby, etc.)? Sometimes kids act out in a negative way because they aren't feeling enough attention. This isn't new news, I know. But in our busyness as adults, it can be easy to forget how much undivided attention they need and deserve. This is also a tangible thing we can always improve!
Lastly, I would put the behavior into a bigger context. It's easy to lose sight of the forest when the trees in front of us seem huge and overwhelming. Try to send up a figurative drone to check out the situation from above. When does the problem behavior happen and is that indicative of anything note-worthy? For example, if it only happens during the school year then perhaps it would be worth exploring: is there a stressor associated with school? Are there academic or learning difficulties? Is your child over-stimulated? Is there bullying going on? Upon analyzing the entire picture, do you notice other behaviors that could be related? Do the symptoms that you see collectively point to a behavioral or conduct disorder, for example? Are we really seeing an anxiety disorder manifest itself in outbursts in preparation of facing triggers?
Every situation is different so determining the root cause of the behavior can be essential in determining the proper course of action or relevant coping strategies. No one knows your child better than you so trust your instincts but also rely on the help of people who have specific areas of expertise.