too broad of a question There's a lot involved. It's like making any other web template - the best way to start is to use an existing theme (or even the default out-of-the-box one) and modify the hell out of it.
That's the worst thing to do actually - most of free wordpress themes are made like that and you can see it (bloated code, often validation errors, unneccessary use of images when CSS could cut it - resulting in slow loading themes, not cros-browser compatible etc). I mean really if you look at the free themes and compare them with the premium ones -there are only few exceptions that stand out but mostly free themes are just much worse. I'm not telling you should start from a scratch but just modifying an existing theme is not very professional. My way: I have a small archive with CSS valid speed optimized CSS templates (coded by me) - I just choose the most appropriate one and start coding it into wordpress theme, step by step, once I have index main page finished I continue with other templates (archives, single etc) then add comments template and so on... Once everything is done - I check the code again and remove any unneccessary lines, minimize the code, make sure both xhtml and css are valid (I'm using strict specification most of the time and level 2.1 css). Then I check whether the theme looks the same in all major browsers, if not I figure out the reason and fix it. I compress all used images so they are as small as they can be, as well as stylesheets and javascript files (if used) and make sure the theme is loading fast. And so on until I'm satisfied with it. Of course there are many great free themes (coded really well) but the problem is that they are used so often - and I just prefer my blogs to have unique themes so I make my own ones - really it's a great turndown if you see so many other blogs around using the same theme. The blog then seems boring and the same as hundreds of other blogs...
well, my guess is that he doesn't know how to create themes at all...so wouldn't it make sense to learn, first?
U need to have expericence in html, css and basic of php. Here is a tutorial how to create wordpress theme www.wpdesigner.com/2007/02/19/so-you-want-to-create-wordpress-themes-huh codex.wordpress.org/Theme_Development
The tutorial that Jalpari has mentioned is great, it will teach you the basics to get a template done and in CSS and XHTML. Seo wise, you can deal with that once the blog is up, using a couple plugins etc. Stylish? well you have to learn that if you want to make them yourself, ive done 2 so far, are they stylish? god no but that will take practise. If you cant do any of this then you will have to pay someone who has the right skills to provide you with a stylish theme. Fast loading is also dependant on your hosting and the amount of plugins, widgets and other things you add.
Yeah. If he just want to learn, it might be a good idea - but I don't recommend it if he wants to make professioanl themes