She will use the Radio( which I think will only be on the 520 ), photos, whatsapp, rarely browsing the internet, ocassional email and calls.
To be clear, a radio only makes sense when you lack unlimited internet access. Not only because you can stream almost anything over the internet these days, but also because you get a lot more radio channels to choose from. For example, if your mother intends to use the radio primarily at home, where I'll assume she has WiFi (without data limits), then using apps like
TuneIn or
Nokia Music are better than a real radio. However, if your mother prefers to use the radio "on the go", where I'll assume she uses a standard data plan (with data limits), then that is where a real radio is useful. Only then should radio support really be a priority.
Unfortunately, none of the current WP8 devices include radio support. None! The GDR2 update is expected to enable radio support for all Lumia WP8 devices except the L620. However, if radio support is important to her, I would wait for GDR2 to be released and get confirmation before buying a L520. The WP7 based L800 already has radio support.
If your mother wants to make photos as well as view them, then the L520 is the better choice. The L800 has the worse camera, despite having the higher resolution sensor (
see this).
Internet browsing will be better on the L520 than the L800 due to the faster hardware. The slightly larger screen of the L520 may also make internet browsing more enjoyable on that model.
For the other things either model should be fine, although I expect WhatsApp to start stagnating on WP7 soon. I don't expect new versions of WhatsApp to come to WP7 devices any longer.