Coughing up phlegm isn't necessarily bad, in fact i think its a fairly natural process (after you've had an infection the phlegm then 'comes up' and its better to spit it out rather than swallow it). But if she's doing it every day then she's either got a permanent infection (which isn't even remotely likely without other symptoms), or she's naturally got a lot of phlegm from smoking or some other reason. It could potentially stip her airway of the protection phlegm (or mucus) provides and leave her more open to infection, but you can't really say whether she's doing that or not. I'd just put up with it, she's probably not doing any harm and it is, after all, her flat too and if she feels the need to cough up phlegm then thats her choice - i mean, where else is she going to do it if not in her own house?