Decayed tooth structure and periodontitis are the common source of halitosis. Seeing a dentist is advised.
Sounds like post-nasal drip, sinusitis, or a candida albicans infection. Do some research on the net, and then tell her to speak to her doctor. The fact that it comes around every couple of weeks might tie in with her menstrual cycle, take note of when it happens and see if its just before/during or just after her period. If its hormonal, perhaps get her to try a birth control pill/come off the pill if she's currently on it? That will change her hormone levels and thus might get rid of the bad breath.
The doctor talking about tongue plaque getting caught in the grooves in her throat sounds like tonsiliths, or 'tonsil stones'. These are hardened formations of bacteria that get lodged and develop in your tonsils. She may have coughed them up before - they're small yellow things and stink something shocking when you crush them! What I do is take a look in the back of my throat every week in front of a mirror. Get her to grab a free-standing torch or find a lamp or something that will give her enough light to see the back of her throat (she might even be able to see the tonsiliths straight away), pull her tongue out and hold it (I suggest using a towel or something), and then using a cotton bud stick or her finger, have a poke at her tonsils at the back of her throat and see if she can dislodge anything. Takes a bit of practice to stop yourself from gagging! Oh and tell her to start using a tongue scraper if she's not already doing so.
Meanwhile, she should stay away from dairy (which thickens mucous and provides the right environment for tonsiliths to form) - that means cream, icecream, milk, white tea, white coffee, chocolate etc etc. Alcohol also dries up moisture in the nasal and throat cavities, causing mucous to become thick and gluggy. Cut right back on wine, spirits, beer etc and also any perfume which will irritate the nasal cillia. If she is going to be drinking, tell her to try to stay as hydrated as possible by drinking lots of water! Maybe get her to try cutting these things out of her diet/environment for 2 weeks and see if there's an improvement.
Oh and has she tried a Neti-pot (saline solution rinse)? These are great for instant nasal relief, flushing out allergens and moistening the mucous membranes.