I have a Dell desk computer and my friend want to buy a Mac mini
computadora. If I send to him an e-mail or use messenger, is he going
to read them?

Dear Guerito, 

Your friend will be **able** to read them. 

A Mac Mini user will be able to send and receive mails and IMs from
Dell desktop with no problem, if he has the proper software installed.
Most, if not all, IM softwares have a Mac version, and so do all email
services.

Naturally, he also has to be connected to the Internet. 

The question if he is **going to** read them of his own will? 

Well, you say he's your friend, why wouldn't he? 

I hope this answers your question. Please contact me if you need any
clarification before you rate the answer.

You can read email no matter what operating system you have. It is a
standard just as much browsing the Internet using any flavor of a
browser you like.

IM's (Instant Messengers) are almost a standard with MSN Messenger at
the top of the list. Now you can connect with to Yahoo Messenger users
with MSN Messenger without any problems, but even if you do not have
access to a specific IM you can still find other IM's that are
compatible with many platforms. Take for example Trillian Basic or
Trillian Pro.

http://en.wikipedia.org/wiki/Trillian_(instant_messaging_client)