I don't know about interaction between those particular meds and weed, but a pharmacist would probably be an excellent person to ask. Don't push her to smoke if it makes her feel bad or if she feels more comfortable not doing so, especially considering the medication she's taking. Isoniazid (INH) is a heavy-duty antibiotic that can cause liver toxicity, and so it's entirely plausible that her body already has all the work it can to do metabolize the Isoniazid without adding weed into the mix.

http://www.nlm.nih.gov/medlineplus/d...r/a682401.html

If your girlfriend has been exposed to tuberculosis, I'm hoping you're taking precautions yourself so as not to be exposed to it, too, from her in the event that she has contracted it. Don't share joints or cigarettes. Make sure any of her coughs are well covered. Ideally, you wouldn't be getting within airborne/saliva-borne illness range, but I know that can be tough if she's your girlfriend. Mostly go easy on her and don't push her to smoke or do anything else that might further tax her body at this delicate time.