Chem over the summer will be very intense. Think 2-4 hours of class each day when you combine lecture plus lab, and she may have class 3-5 days a week (check the schedule to see how the days / times play out). Homework and studying may take another 1-4 hours / day.
Chem at the HS is a 1-year class. DE Chem is a single semester, so all of the material in roughly half the time. DE Chem during the summer is roughly six weeks, so all of the material in less than a quarter of the time. Missing a single class due to travel or illness is like missing an entire week of HS. Missing 2-3 classes might get her dropped or failed for poor attendance.
Have her contact the professor to ask for a syllabus, then go to the bookstore and spend some time thumbing through a copy of the textbook. Use the syllabus to see how much of the book will be covered, including how much per week.
Be aware that her DE Chem class may or may not transfer to her ultimate selection of a 4-year college or university. It depends widely on the individual CC and the target 4-year school. Is she willing to take the class again if it doesn’t transfer?
Note that I’m not saying not to do it. I’m saying that there’s some additional investigating to be done.
Re: easier vs. harder - My S looks at it this way. Taking an unpleasant course at the HS level is like hitting your big toe with a hammer every day for nine months. Taking the unpleasant course DE during the summer is like hitting your big toe with a 5 lb. sledge, but you only have to do it 6 times. For him, it’s not easier vs. harder; it’s which variety of pain he finds more palatable. 
(We refer to this as the PITA factor.)