How much data will this use?

[GLaDOS]

-Web Browsing-
Each page has variable size to load, but most of them don't go over 100kB. According to Google, the average web page is usually 40kB.
400 Web Pages would therefore be around 16MB.

-Facebook Pages-
Facebook images are usually compressed for a smaller filesize, but with all of the content on Facebook including the images, you're probably looking at 500kB per page.
100 Facebook Pages would therefore be around 49MB.

-Skype-
Microsoft said that the data it takes for just messages is very little; less than a kilobyte (kB), but skype calling will maximally take 100kbps upload and download, and video-calling taking 128kbps upload and 512kbps download.
2 hours of Skype would therefore be around (at most) 450MB.

-Emails-
Emails are also variable in size, with plain emails being less than a couple of kilobytes and those with images being up to 5mB (on average).
100 Plain Emails would be about 0.4mB.
100 Imaged Emails would be at most 500MB.

-Apps-
Apps on the App Store are usually about 20mB, but can go all the way up to 100mB, at least for the common ones. There ARE apps out there that is more than 600mB!
But we'll say that for the most part, she will be using at least 400mB.

Totalling all of this, your relative would be using at least 1.38GB per month.

Meaning a 1GB plan will not be enough for her.