the traditional way to import is to plug a cable in from phone to computer, and there is, beside the multiport adaptor, a cable which does just the usb-c to usb so she could, with this $19 cable, plug her Iphone into this cable using her standard iphone-usb cable and import the pictures into the "photos" app. http://www.apple.com/shop/product/MJ1M2AM/A/usb-c-to-usb-adapter
When I mentioned getting files from dropbox, she would have dragged the files from the dropbox folder into the Photos App, not a place but an application.
I do see where One-drive app is available for the iphone and can auto-import.... mine is "hung" at the moment "finding new photos". I believe if she has subscribed to office 365, she gets a terabyte of storage. If she has an email with Hotmail/Outlook.com, she gets 15gb of free storage. One caveat.... one drive does not act like dropbox with many file utilities, it is a special connection that mimics a folder, but many programs that use normal folders don't quite work with the one drive interface. In addition, I had uploaded several gb of pictures (luckily I made a copy and didn't just MOVE them into one drive) and when I went back later, a lot of the important wedding photos showed corrupted. I tried several things to "redownload" them to my computer, but that never worked. I had even "removed them" and tried to reset it up on the pc, but the redownloaded photos also showed as corrupted, though I could see them on my phone. I understand it gives you more space, just not sure I trust it all that much. (my photos are finally going into one-drive from my phone, but i can't tell how it determined which ones to upload, as I have 11000 on my phone, and it only has uploaded about 1900 and says it's complete.
Photostream is a special apple online storage "pipeline" that iphones and ipads can use and macs not using "icloud photolibrary" can use and the photostream shows up automatically as another album in the Photos app separated by date, like Photostream august 2015, etc. However, after you convert a device to using the Icloud photolibrary, it no longer recognizes photostreams. The problem with photostream is that there was a maximum number of photos (1000) and time (30 days) that it would keep these.
My Photo Stream and iCloud Photo Sharing limits - Apple Support
Bottom line, i'd get the cable and import photos weekly into the Photos app. then remove from phone ones she isn't interested in keeping on the phone. And wait until she has a phone with enough capacity to turn on the Icloud PhotoLibrary when she has a phone with about 5gb free.