Negotiating down medical bills in CA?

My girlfriend was on her athletic coach’s individual family insurance plan through Cigna for several years. Her coach (another woman) is the primary policy holder, and also legally adopted my GF into their family as an adult, which apparently you can do in CA (GF used to have a really shitty relationship with her own parents). Over the last two years my GF needed a few MRIs and X-Rays, and apparently, her coach/her coach’s insurance never paid any of the bills for those. I don’t currently know what the co-pays or the deductibles should have been while she was under that insurance plan but now she’s getting bills directly from the providers. I’m not sure if those bills are for the total costs or whatever her co-pay/deductible should have been. But one of them was $800 for an MRI.

She and her coach had a massive falling out six months ago and they separated. My GF is back living with her parents. But, the adoption wasn’t undone and she’s still listed on her coach’s insurance plan, though her benefits and coverages were slashed to almost nothing. She’s gotten two or three bills directly from her providers up until now, which she was able to pay, I helped her with one. She’s panicking because she can’t handle those other bills on top of her other expenses right now. She has a new job, but she doesn’t make enough to live on her own yet.

How would one best go about lowering these costs? What are some strategies to pursue or steps she can take? Can she negotiate with the providers directly to lower the costs? She says she’s tried to make claims through her insurance for some of this stuff before, to no avail. What can she do there? Are there any paid professional services that specialize in this sort of negotiation?