Simple answer. All crystals work by balancing so there can never be a “dangerous” crystal – except those that have physical warnings such as cinnabar – wash your hands very well after contact with the rough material before eating because it contains mercury that can easily be released to your skin - it's perfectly safe as long as you don't eat it! I would ALWAYS recommend malachite as a general crystal for the heart chakra.
 
I’ll be blunt, there are three reasons for this warning you have encountered. Firstly malachite works very effectively on the heart chakra and anyone out of balance here will have some reaction to it and some people may have had big emotional releases – but this is good – not bad. Secondly some importers will spread rumours to suit themselves and there have been two times when malachite has been difficult to source. In the late 1990’s and early 21st Century because of the wars in the DRC (Congo) and now because the Chinese need copper and malachite is a copper ore – the price has recently more than doubled. Thirdly the person has no understanding of how crystals work. I said I’d be blunt!
 
It would be interesting to know why you call the client “toxic” – how were they poisonous? In character, energy, spirit, disease? But my guess is that the crystal either wasn’t cleansed before the treatment – which would confirm your energies were not compatible with theirs, if nothing else or it had been overworked with you beforehand. Either way put it back in the earth and buy another for yourself and perhaps a separate second one for you to work with clients.