I sell digital files all the time. It's no big deal and demand for it is increasing every year. But there are some very important considerations and licensing procedures.

First question is, what is her intended use? Personal or commercial?

If she just wants freedom to print them herself, using the printer of her choice to make a big print to display on her wall at home (and even reprint in the future), that's personal use. Displaying a smaller version on a personal website or sharing it somewhere online also would be personal use. I license this type of usage perpetually, non-transferrable and non-exclusively, with copyright remaining mine. I never sell "all rights" or ever, EVER transfer copyright ownership.

For personal use, I offer two sizes: "Low resolution" that are plenty for 4x5 or 4x6 prints, maybe 5x7 or online sharing and display (1200x1500 or 1200x1800 max), $8 apiece.. And "high resolution" big enough to make 8x10 to 8x12 size or slightly larger (usually about 2400x3000 or 2400x3600 pixels) for $18 apiece. If larger number of digital files are purchased at the same time, I offer some volume discounts, too. All images sold this way are "marked" with a small, unobtrusive "signature", with copyright info and license details embedded in the image EXIF and, optionally, as a plain txt file. The license also allows for third party to provide printing services or host the image on behalf of the licensee. Any other usage, transfer of usage rights, or modification of the image other than minor re-sizing requires additional, written approval of the the copyright owner (me!), which would only be given after negotiating additional payment.

30x40 inches @ 300 ppi is huge, though... and really makes me kinda suspicious that she might have a more commercial intention with the images. I'd be asking more questions. Lots more!

Using the image in one calendar to hang over her desk = personal usage. Using the image in a press run of 100 or 1000 or 10,000 calendars that will be sold = commercial usage. Using the image to make a dozen or twenty postcards to send to friends = personal usage. Using the image to make 10,000 or 100,000 postcards to sell = commercial usage. Using the file to make a 30x40 print to frame and hang on her living room wall = personal usage. Using it to offer up to 30x40 prints for sale on certain websites or in a local gallery = commercial usage.

Rates for commercial usage (including one-time editorial usage) are higher... much higher in some cases depending upon the type of use, quantities, any sort of exclusivity, rights of renewal, etc.

If her intentions are commercial, far more details about exact use of the image needs to be known, before any sort of price could be negotiated and determined. It could be structured as a single fee, up front... or as a per-item royalty on sales... or a combination of the two.

30x40 inches @ 300 ppi is doable... as noted up-rezzing will certainly be needed, but I disagree with previous response. I think that, assuming the image itself is a high quality capture, with care it could be done. However, for printing purposes 300 ppi is generally overkill. Printers I work with are happy with 170 ppi for prints that large... 5100x6800, or about 35 MP. There would be no discernible improvement in quality, merely by providing 300 ppi. vs 240 ppi... and little to be gained over 170 ppi with many printing processes for a very large print that will necessarily be viewed from some distance. $50 seems reasonable, for personal usage only such as I license, and considering the extra post-processing work that would be needed to make a 9000x12000 pixel file. If you end up making it that large, it might need to be burned it to disk and delivered or mailed, due to the size, rather than sent via a download (something like Dropbox might work... can't say for sure since I've never had reason to deal with that large a file).