Is it perhaps that they are denying coverage for BOTH medicines? That is, if she would be on just one (either one) they've cover it, but because they're being asked to pay for BOTH of them, that they're balking? If that's the case .. she should drop the Avonex for now, go on the IVIG, and then appeal to Avonex to cover their costs. They do have a fund to help with that sort of thing.
Unfortunately the way the insurance companies work these days is that they don't see patients, they only look at the bottom line. . the dollars. I'm sure somewhere there is an actuary that says it is "not cost effective" to pay for both meds, that there isn't sufficient evidence to support that treatment..at least not sufficient evidence from the insurance companies bottom-line point of view! That sucks, to be sure. And they can (and often do) change their minds about what they'll cover..and not cover. Sometimes they won't continue to pay for meds for folks for drugs they'd covered in the past but have decided to no longer cover. And again, that sucks. I'm sorry!