Good luck to your friend!
Informal recruitment can be done a few different ways, depending on the school, the chapters, and how many bids each chapter has to offer. If a chapter only has a couple of spots to fill, they may just invite a handful of women to some events and choose from among them. If a chapter has more bids to offer, they may advertise as well.
Informal recruitment events are usually more laid-back. You don't have the same strict scheduling as during FR - if an event runs from 7-9 and you show up at 7:30, that's ok. If a sorority has 3 events and you can only get to 2, that's ok too. We've done mocktails, cookie baking, picnics... unlike FR, the sororities aren't restricted to their houses or rush rooms.
Pref may or may not take place. I think that if it does, it's usually invite only. My chapter did not do pref during informal rush.
As with formal recruitment, your friend should look neatly put together, though she probably doesn't need the latest from Lilly.
And she should try to relax, be herself, and have fun!