Why would he have to do the server in another language? Unless he wanted to use a linux server, then he would only be able to do it in BlitzMax. You can make a decent server for a small player base with blitz3D just the same as with plus.
Latatoy, just program your server in blitz3D, then if you want to get a slight speed boost then you can just compile it in BlitzPlus (the code will work, with probbaly no alterations). If your game gets really big, then you can worry about getting really good speeds from your server side stuff, and have the money to hire some good c and asm boys, and buy equipment to have server clusters. Until then keep it in the language you know.
Just keep the server lightweight, don't bother using any processing loops on graphics stuff for the server, just simple text that you can use to debug, and that can be turned off (so its just a black running window).
And to be helpful, have a look at some already made networking libraries:
http://www.rottbott-studios.com/rottnet.htmlhttp://www.blitzmax.com/Community/posts.php?topic=75705