Transferring photos from iPhone to iCloud to free memory.
My daughter's iPhone 6 Plus has 128 GB of memory. She has completely filled it with photos. I got her on her husband's iCloud account and we have elected iCloud on her phone and Optimize iPhone Storage. We also backed everything to the iCloud using iTunes on a PC. After all this she still only has 420MB of free memory on her phone. She is leaving a vacation tomorrow morning and wanted to obviously take a lot of pictures with her iPhone on her trip. How can we get some of the memory back now that their backed up to the iCloud? Apple support says Not to delete any pics from the iPhone or they're gone, the Apple Store says you have to delete them from the iPhone. We just don't know what to do! Your help is needed!
iPhone 6 Plus, iOS 9.2.1