No doubt you will get some votes for a Northern or Northwest suburb, and they ARE great, but if affordability and a shorter commute are top priorities, I highly suggest a Western or Southwest suburb.
Affordable Western suburbs that I really like are: Clarendon Hills, Willowbrook, Darien, Westmont, Downers Grove, Lombard. Parts of Clarendon Hills and Westmont are in the Hinsdale Central school district, which is one of the best in the state. Hinsdale itself is REALLY pricey, but if you find a house in Westmont that is in the district, you'll be getting a GREAT deal on the schools.
Here in the Western suburbs we have some great malls. Oakbrook Center (about 20 miles west of downtown) is a gorgeous mall with great restaurants, and is an outdoor mall. We also have Yorktown about 5 miles west of Oakbrook and that also had great restaurants, is indoors, and has a great play area that I love to take my kids to.
Western suburbs that are a bit closer to the city that I like are: La Grange Park, Westchester, Willow Springs, Countryside, Indian Head Park
I'm not certain about the schools, but I think they are good.
Southwest suburbs I am not as familiar with, but I have heard really great things about Oak Lawn.
Another benefit of the western burbs: it is about 30 minutes to BOTH of the airports, Midway and O'Hare, which makes it very convenient for travel.
Good luck with your search, and if you have any more questions, just post or ask in PM. I have lived in the area since 2001, and have lived in 3 different suburbs, as well as along the lakeshore in the city.
ETA: We have a really great public transportation network, so he could take the Metra train in. There are 2 different stations in downtown Chicago, Union Station and Ogilvie. Does Northwestern offer a shuttle from their campus to the Metra? They probably do, as my university (Loyola) did. That made the commute very affordable and shaved off some time that would otherwise have been spent catching CTA trains/busses. You might want to start looking at the schedules for commute times so you can see what would be too far. http://metrarail.com/metra/en/home.html There are some express trains too :) If he is intending to drive, parking downtown is ATROCIOUSLY expensive (thinking $20+/day).
ETA2: Are you looking to buy or rent?
ETA3: Another thing to consider is how dense of a population you are comfortable with. When I lived in the NW suburb of Des Plaines, it felt like there was a BIT of elbow room. Closer in than that and it feels very squished. In the Western burbs there is quite a bit of extra room.