Your friend is correct about Dayana and Daianna. For a Spanish-speaker, these approximate the English pronunciation of Diana. As another example, Brian is sometimes spelled Brayan by Spanish-speakers.
It is also correct that in Spanish, Diana is similar to dee-ah-na.
Deyana and Deiana, however, would be pronounced in Spanish something like "Day" (the English word)-ah-na.
This message was edited 8/11/2010, 8:45 PM