MacOS – Mavericks or El Captain for old MacBook Pro (early’11)

I used to work with a 2009 MBP 13" with C2D 2.53Ghz, 8GB ram and 256GB SSD, it worked like a charm with development (Qt, XCode), VM (VirtualBox), image edition (lightroom).

I changed it because the screen was too small but that shouldn't be an issue for you since you're running on a 17". So if you change both, you can hope at least 2 more years.

I would go for the SSD which is awesome : your HDD should reach around 100MB/s in RW whereas the SSD can go up to 375MB/s in SATA2 which is very nice. You'll get more speed in loading softwares, and swapping will be faster in order to compensate the eventual lack of RAM.

Your logic is sound. Just something to keep in mind is that while yes swap is much faster on a SSD, it is still much slower in respect to RAM and both should probably be upgraded. But yes she would see the most improvement by going to a SSD

Just as a side note 10.10.1 will try to pre-cache as many things as possible in RAM so apple uses 'memory pressure' to reflect the systems demand for RAM.