I really don't see a reason your friend
shouldn't try playing Morrowind.
OK, now a bit more serious answer.
I think you should be wondering about a different thing and that is
if Morrowind is the game you want to base your mods on. The main question is:
What do you want to make? Do you want to make something just because you feel like it, or do you have a certain idea in your head?
If you don't have a clear idea yet, just want to try make something: The question is, did you already mod for some other games or is this your first try? The TES series games are a good choice for a first project, since they have a large thriving modding community, plenty of good tutorials for both, beginners and advanced mod makers and of course people willing to spend their time and think of solutions for you, when you get stuck with a problem. The Construction Set is a rather easy to use tool too. Take your time and do some beginner tutorials step after step to gain the basic knowledge and feel for the CS. Once you do, you can start thinking about what you would like to add to the game with it, or what kind of your
own game you would like to make.
If you do know what to make: Is the Morrowind engine and gameplay fit for it? Will you be able to realize all your ideas with it? If you answered both questions with a yes, then don't think about it too much and start learning the CS. If you aren't sure about it, a good idea would be to look at already existing mods from other modders. The modding community knows how to get the maximum out of the game, making the seemingly impossible into reality. There exist tens of thousands mods for Morrowind, from simple stuff to complex total conversions, completely rebuilding the game from a zero state (
http://www.gamesas.com/index.php?/topic/1067645-relz-arktwend-english-31/page__st__140__p__16741470__hl__arktwend__fromsearch__1#entry16741470 comes to mind, it includes a completely new world) or building a new story on the already existing one (like
http://planetelderscrolls.gamespy.com/View.php?view=Mods.Detail&id=2517).
In both cases, since you have played the game some time ago, it may be a good idea to refresh the memory of it a bit by playing once again. And since you want to make a joint project with your friend, he should try Morrowind too, so that you can both get to know it a bit better and brainstorm for your project. For a comprehensive knowledge about the lore you may check out the
http://www.uesp.net/wiki/Morrowind:Morrowind and for tutorials check out the
http://www.gamesas.com/index.php? here on the forums. One last advice at the end - start small and don't try to swallow all information and knowledge at once. Take everything step by step, so that you don't get overworked and end giving up even before properly starting.