It is not infertility treatment, it is a potentially life-threatening result of infertility treatment, so her insurance should not balk at paying the claim according to the policy - unless you have something else written into your contract. If her insurance policy has a typical 80/20 ratio, then insurance covers 80% of the cost and you would have to pick up the remaining 20% of the cost.
If she is being monitored as an outpatient, then the costs will be minimal. If she goes into the hospital, then the costs could balloon. You didn't mention her being admitted and I assume retrieval was at least three days ago, correct? This next week is critical but usually OHSS resolves in the absence of progesterone. That's why when a woman doing IVF with OE develops OHSS and then goes on to transfer, with the continuing presence of hormones during the 2ww and more if she does get pregnant, it can get really dangerous. In a donor's case, though, they will pull her off all the hormones and hopefully all will just settle down. Sometimes they choose not to even trigger the donor because the trigger shot itself is the real problem as it causes release of the follicles, stimulating progesterone production and causing fluid leaks into the peritoneum.
My first donor hyperstimmed and she did not need to go in the hospital. She also did not receive the trigger shot though as they were so concerned about her they didn't want to take that chance. Basically in her case the follicles just shrank down and were reabsorbed by her body. There are varying degrees of OHSS, as you know, some mild, some severe. Your donor will be dealing with the trigger shot part of it, but she won't be getting pregnant. She's kind of in the middle ground, in other words. She got the HCG trigger shot, worsening the OHSS, but she won't be getting pregnant and with her progesterone and estrogen dropping dramatically, hopefully she will just push on through it without serious complications.
Congrats on getting so many eggs though. I know it's hard to focus on that right now.