Thre is a specific blood test that is run to determine whether she has developed antibodies. But generally the development of antibodies happens in the first year or so...and you say she's been on Betaseron for 6 years, now...so if this was to happen, it would have happened in the first year or so. Also, there is some evidence to suggest that even if the antibodies do develop in that first year or so, they may not actually affect the efficacy of the drug. That is, it may not matter, and the drug may still be effective.
If she had a base-line MRI done when she started taking the Betaseron, a new MRI now might show if new lesions have developed. But of course, NONE of the drugs has promised to stop ALL exacerbations. She may simply be experiencing an exacerbation, which would have happened with or without taking the drug.
You say your mother has had MS for 25 years. It may simply be that she is declining. I've had MS for 23 years now, myself. At age 59, it's hard to say if the decline I'm seeing is because I'm simply getting older, and/or if my body is just wearing out becuase of having to deal with a chronic illness for that long, and the disability it brings, and/or if the MS is worsening (I'm secondary-progressive; your mother apparently is still labeled relapsing-remitting, a GOOD thing after all those years!)
What are you wanting your mother's doctor to do? If she is experiencing a relapse, a possible treatment is steroids, but there has been increasing evidence to suggest that they often do more harm than good. Many doctors choose not to prescribe them unless the patient becomes unable to "perform Activities of Daily Living" -- ADL's -- like, she is no longer able to walk, or her legs won't support her, or she loses bladder or bowel control, or suffers severe vision loss, or is unable to feed herself, or dress herself, or bathe herself, etc.
Presumably you're in contact with your mother, and with the doctor, and if things worsen to the point that she isn't able to care for herself sufficiently, then more drastic measures might be necessary.
It's good that she has such a caring daughter as you to help her monitor the situation. I'm sorry about the anorexia; that can only complicate matters.
...I am not a doctor, nor health professional, and don't pretend to be one, here.....