Dear Sonja,
Tina Ostreich covered this very topic nicely in the most recent IALLT Journal:
http://www.iallt.org/iallt_journal/llti_highlights

Desktop solutions (Audacity, Garageband, DL Recorder, etc.) will always give you better quality and these days, with spacious email accounts, may be all that your colleague needs. She can record her prompts as one file and send them out to students. And they can respond in kind.
For more "natural" conversations, the Voice Recorder on iOS allows you to send your recordings directly via email.

For large scale work (e.g., multiple sections of the same class), a web-based solution makes a lot of sense. Our own ANVILL, for example, allows teachers to set up Voiceboards that make the process of assigning and giving feedback very simple.
Tina's article covers the spectrum of low-cost or free choices available.

Cheers, Jeff