Lived in Chicago 50% of the year for almost four years. Summer is great, but it will be cold as balls when the winter hits. Think hypothermia warnings as part of the daily forecast. It's a fantastic city to meet people as well. Most are pretty friendly and most her age will be expats as well. Pretty much anywhere on the north side of town will be fine as far as where she's living is concerned. Pretty much anywhere in this area will be perfectly safe, connected to the rail lines to get you into downtown, and can also get you to both O'hare and Midway in about 40-60 mins. Attached a screenshot of a map including neighborhoods for reference.
As far as cost of living is concerned, it can be a bit misleading. You don't really need to drive anywhere in Chicago due to the rail system being as comprehensive as it is. Most people do a ton of walking there. There isn't really a need for a car. It's more of nice to have - and usually you have to have a rental with parking including or you can wind up paying someone else to park in front of your own place.
Groceries really aren't that different either. Jewell's is about the size of a small HEB and will have everything she needs. If you want to be a big help to her, get her a down coat and some winter boots. Everyone there has a down north face and sorrels for a good reason. Tell her to check out wicker park, bucktown, lakeview and lincoln park for people her age. If she want's something quieter and more chill Roscoe Village is a nice spot to be. It's where most of the athletes from Chicago teams live if they're not out in the burbs. River North is full of the 40k millionaires but has a decent food and bar scene.
Three dots and a dash is a great tiki bar in the basement of a building there and has drinks that will fuck you up three ways from Sunday. The aviary is overrated. Sushi is generally pretty good. Water house on Sundays is great for build your own bloody's and football crowds. Depending on her budget, I can send you some specific buildings that I know are in nice areas and have decent amenities from friends who are still there.
Chicago is a great city all around. She should be pumped.
PS. Her uber bill might get retarded high but it's still cheaper than cabs.
PSS. It's only about 2.5 hour flight there and a 2 hour flight back via southwest and tickets can be dirt cheap. $99 each way unless it's the summer.