Mixing various vegetables and fruits - Not Good?

My In-Laws are in England and they called today to ask me this question regarding mixing of all varieties of vegetables and fruits. They were told by a friend that this is not good as some fruits and vegetables should not be mixed. They have been juicing for a year now and are very happy with the results. They are in their seventies and quite happy to see their bodies rejuvenating.

Any thoughts on cross contamination and toxins from mixing?

Thank you.
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