There is no specific dieat for Mitral valve prolapse
. First to reassure you Mitral valve prolapse is not a serious condition and 5-10% people can have it. Some time when it is associated with Leaky Mitral valve then it needs regular follow up and may be treatment.
As far as diet is concerned , people with Mitral valve prolapse tend to have symptoms of palpitation
which gets aggravated if you are drinking too much of Tea, Coffee , Alcohal and Coke . So these things should be taken to minimum if some body has got symptoms.