Yes, definitely a myth. It is best to stay with the same diet and habits as before seeing a homeopath, as it will be difficult to know what has caused what. For example, if you drink a lot of coffee and you give it up just before you take your remedy, it will be difficult to know if the well-known irritability and headache that some people experience when giving up coffee are due to the remedy or to the lack of coffee. A well-known homeopath used to offer coffee to her patients at the end of the consult, just to prove this point.
Having said that, coffee is inimical to some remedies, meaning that coffee “fights” with some very distinct remedies. The homeopath can easily check this out.