How to update iOS with limited hard drive space?
A conventional spinning hard drive on a computer needs to have at least 15% free space for optimal operation. If a spinning hard drive has less than 15% free space, the entire computer system will run slower than it should, due to a phenomenon called disk fragmentation.

Your friend will do herself a favor if she buys a larger hard drive and pays a technician to install it and clone the contents of her older, smaller hard drive to her newer, larger hard drive. Then she will not only have a faster computer system, she will also have plenty of room for iTunes and the iOS upgrade.

Large hard drives are ridiculously cheap.

That being said, there are many things that can be done to clean out un-needed files from the existing hard drive to free up more space. Start by running the free CCleaner program.

Then free up space on the disk by deleting all but the last Windows System Restore Point. Do a Google search for instructions on that.