Loyalty, tact, and discretion

Two friends, Danielle and Kami are on a holiday in Hawaii, and together they end up meeting and chatting with a charming fella, a fellow tourist. Danielle gives her e-mail address, and two weeks later, to her surprise, the guy sends her a friendly e-mail.

QUESTIONS:

1. Should Danielle mention this to Kami, knowing that Kami may be jealous and/or may ask her for the guy's contact info?
   
2. If Danielle does tell Kami and Kami asks for the guy's e-mail address, what should Danielle do?
   
3. Would your answers be any different if the gender roles were reversed (two guys and one girl)?
   
4. Exactly what issues are at stake here?

I'm very eager to hear your take on this scenario, which is indeed based upon a real situation.