Sure. Just email them any MOBI file, and they can open it right from the email attachment.
But Kindle is a truly bad software to read books in on the iPhone or iPad.
A better idea is first to convert the MOBI book into EPUB, so that your friends can then enjoy the book in superb e-reader software like Marvin or Stanza.
Converting any book from MOBI to EPUB only takes a few seconds in Calibre. There are also free online converters, like this one:
www.2epub.com