It's not clear exactly what kind of advice are you after, but as someone who has visited ORD half a dozen.or so times in the last couple of years here goes:-

Firstly, as with any other US airport, get off the plans asap and get to immigration asap. Although automatic kiosks are now in ORD they are supposedly for people returning to USA on an ESTA so your friend won't be able to use them. That means they'll have to join the queue and are at the mercy of circumstance. If they arrive just after a couple of flight from non ESTA countries they could be in for a long wait, and by long my record is 1h58m at ORD so my advice is to mentally prepare for over an hour then anything less is a bonus. They can estimate how long the wait will be by where the back of the queue is. If it it half way through the snake it's about 30 mins, if it is at the end of the snake it's about an hour and if it's out of the snake and round the corner and down the corridor and round another corner it's 1h58m! Once through immigration there might be another queue at customs but it won't be anywhere near as long as immigration and is often just a few minutes. Having said all that the queues could be very short but there's really no way of knowing in advance.

Getting into town (again, you have not specified that you want or need this advice) is also poor in ORD. The freeway between ORD and downtown is regularly at a standstill at peak times on weekdays (you haven't specified what day they're travelling which would help people give advice) so I would recommend getting the CTA blue line which takes about 45 minutes to get to the loop assuming that's where they're heading.

You haven't said if they're on business or please (or both) but my advice is to enjoy one of the best cities in the USA. Avoid the south side but get out of the centre and visit the lovely northern suburbs such as Lincoln Park and Wrigley. I would recommend walking down the beach and through the park to get to Lincoln if the weather is suitable, which in October it should be.