My few of my friends eat pork. I have shared my food and drink with them, from the same plate. I have also shared cigarettes with them (I'm in process of giving up smoking). Is it forbidden or allowed to do so?
It should be fine. As long as the plate does not contain pork. Eating pork is haram, so unless the pork is making its way into your mouth somehow I don't see how you will be eating the pork. Anyone who tells you otherwise, please make sure they have some clear Hadith to tell you so (there aren't any unless they take a totally unrelated hadith and make some out of the view interruption). The prophet PBUH sat and ate with Christian and Jews (from the many hadiths we all know).
According to some scholars (for example the Malikia), a Muslim Husband can not disallow his non Muslim wife from eating pork or drinking wine, but needs to ask her to wash her mouth before kissing or having sex.
Now in regards to smoking, the majority of scholars say it is harmful thus haram. I would advise you to attempt to quit it as it hurts you and wastes your money.
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My advice is to stay away from pork. You seem quite clued on. It's not difficult to understand what's right or wrong. Keep your faith, strongly respect your beliefs, and understand who you are. In this society we have to work hard to behave, and at times it's hard. Always keep Allah the all mighty in your mind and behave in a Islamic manner. Forget the rest, the only real friend or job you need is Islam. That's were you will earn more respect and rewards then any non-Islamic human being.