At one point my daughter was thinking of going for her PhD. If I understood her correctly (and maybe I didn’t), it seems you need to have research experience and an understanding in the area that you wish to obtain the PhD. You also need to contact labs that have your area of interest, and connect with faculty from those labs. It’s a matter of finding the right lab to suit your interest, if I understood her correctly. I would imagine that different labs/programs have different requirements in terms of experience (but I do not know)? Some may have more leeway.
I think that some PhD programs have you join a lab right away, while others allow you to join later on. It depends on the program. My daughter has friends from college who went straight into their PhD program. Both of them had specific research interests and experience during undergrad.
I think these programs are quite competitive. I also seem to remember her telling me that gpa is not what is most important.
Is there somebody at your daughter’s job who has a PhD? That person might be a good resource. She can reach out on her own to specific programs and ask questions. Another possibility is to get a masters in clinical research.