ET's Lauren Zima caught up with Siwa at WE Day California, airing Aug. 17 on ABC, where the 14-year-old star -- who appeared on two seasons of Dance Moms -- revealed she visited Miller on Wednesday.
"I actually went down to the hospital to see her," Siwa shared. "She's not good, but she luckily is in good spirits. She's not 100 percent on what's going on right now, which is not the bad thing, she's definitely in good spirits, but it's so heartbreaking to see what's going on. It's so, so heartbreaking."
Siwa definitely has a lot of compassion for Miller, and says that the 51-year-old reality star has been holding back her tears.
"She is very sick, she's very ill," Siwa said. "I don't know if it's officially announced that she has it, or she's keeping that private, but what she has is a very bad thing and you don't want anyone going through that. I was just telling her, people see her as the scary lady who yells at kids on TV, but it's not true, and she is a person first of all, and she's loving. She is caring, she taught me so many things that I'm so grateful for that it's so hard to see her go through that and being sad, and just laying there and trying to cry, but physically not being able to. She just kind of, like ... she would start to cry ... and then she would just stop."
Siwa reiterates that despite Miller's tough reputation on TV, she definitely has a softer side.
"I think people think that she is just that person on TV ... and no, Dance Moms is by no means scripted, but it is also a TV show, so you do amp it up and you do get a little mad, but behind that she is a loving, caring, nice person and she really, really is."
Miller was rushed into surgery in the early hours of Tuesday morning after several days of suffering from extreme pain in her back, arms and neck area, her orthopedic spine surgeon, Dr. Hooman Melamed, told ET after the operation. Performing a CT scan, the doctor discovered a mass infection putting pressure on Miller’s spine, then rushed her into emergency surgery to remove the infected mass.
Dr. Melamed told ET on Wednesday that Miller's cancer diagnosis is preliminary, and the results are still pending a pathology report. Burkitt lymphoma, which is a non-Hodgkin lymphoma, is a form of cancer that affects the lymphatic system and starts in antibody-producing B cells, which are part of the immune system.
“People who have been diagnosed with this have a good prognosis," Dr. Melamed said. "We’ll know more details soon."
For more on Miller's health, watch the video below: