You're not going to find an IDE for Windows that supports iOS development using Objective-C and Cocoa Touch. Your best bet would be to run Lion using a VM (virtual machine). However, this is illegal to the best of my knowledge. Yes, since Lion there have been some changes to the EULA (End-user license agreement), but these only allow Lion to be run on VMs if the host machine is already running Lion. That's right: you can run Lion using a VM if you're already using it. Not very helpful. Some people would whip out the old "who's going to catch you?" stuff, but that's not exactly sound legal advice.
Your friend has a few (legal) options. They can buy the cheapest device capable of running Lion and the latest version of Xcode. Alternatively, or in the meantime, if you're only doing basic stuff, you could have them write out their code into any text editor which supports Objective-C syntax highlighting, such as NotePad++. Then, they can send the files to you and you can add them to a project in Xcode. Of course, this means you'd probably have to do most of the nontrivial debugging and all of the compiling, etc, and that might be harder on beginners who are more prone to make mistakes. There's really not much you can do. You could look into GNUStep but that's closer to Cocoa and not Cocoa Touch (you said Cocoa, but since you want to make iPhone apps you obviously mean Cocoa Touch, which is different).