I don't know about Linux and MIDI specifically and have never used any sequencing software like Cakewalk or Cubase under Linux.- all I used was whatever default player Mandrake came with - I think the one I used was a KDE one.

If notation software such as abc is on the list of interests, some of the software I mentioned above could prove useful - it's open source C stuff so even if a suitable binary could not be found, compiling with gcc is an option. You could also run skink, a java abc program. The simplest pointer for that area would be the abc home page where a lot of software choices are given. Other musical areas that may be of interest to your friend could include Lilypond and musicxml.