If the internet speeds are so slow, she might have more luck by directly plugging her iPhone into a USB port and downloading the new photos by a USB connection. And uploading selected albums to her iPhone by syncing them to the iPhone with iTunes.
Which model is her iPhone? If her iPhone can take Live Photos and Portrait mode photos, and she likes to take videos, My Photo Stream would be a bad choice anyway. My Photo Stream does not transfer Live Photos as animated, only the still frames. Some users had problems with Portrait Mode photos, and videos can also not been transferred. If the iPhone is a newer model, supporting Live Photos, the direct download by USB will be better.