Maybe the meds she's on has a supression of menstration side effects or it could supress the production of estrogen, or even the thyroid could be effected because it can control libido/ in concern of the sexdrive, and it could be both. I'd consult the pharmacist or a physicians desk reference for implications of the medication and known side effects. I've found research always helps.
I was without a period for probably 6 months and then I started getting my period for 7 days every 10days. They did a ton of blood work and found nothing wrong so the best advise I can give to you is just take her to a GYN and have them run test. My doctor put me on YAZ and it has made me regular and I love it! I hope this helps!