An interesting fact: back in the days when women were not allowed to wear tallits the would often wear knitted shawls both in order to emulate wearing a tallit and because the guys couldn't tell them that they couldn't wear shawls!
As Jeremy says, a lot depends on what your friend wants. A traditional tallit is white with blue horizontal stripes near the ends. But if your friend is a funky sort of gal, then you can have some fun with it! You can really do anthing; I have a friend who has a gorgeous tallit which was made in a very open, lacy pattern with a cream colored yarn.
One other thing which a tallit must have, in addition to the fringes at each corner (and it is a very good idea to reinforce the corners where the fringes are), is a band on the top called an atarah. It traditionally has the prayer over the tallit written on it but I think that the writing is optional. The atarah is usually (very roughly) only about four inches wide and 2 and a half feet long.
You also might want to consider what dimensions the tallit should be; some people like thin tallits; about a foot wide or so. Others like really really wide tallits, the width of a regular shawl or even several feet wide! As for length, I measured my tallit and it is 5 feet 7 inches long (which, interestingly enough, is how tall I am!); that is a good length.
There are also other options besides a tallit if I have scared you by this message. A tallit bag is a wonderful present; it's usually at least a foot square but usually somewhat bigger. Also, a knit kippah is always a wonderful gift (see this thread: http://www.knittinghelp.com/knitting/forum/viewtopic.php?t=28641&highlight=)!
Good luck with everything!