Media Query display issues on one (single!) device

Here's an interesting one.

I've just been working on a site for a client and run into some issues with the display on mobiles devices. I say mobile devices, that's not technically correct; as it's just one single device, which happens to be my clients.

She is using an iPhone 4S which up until yesterday was running 8.1.2 (now 8.1.3), which happens to be the exact same phone as mine. I have absolutely zero issues and the site displays exactly as intended, across all devices, even back to the 3GS on iOS 4.

However, her iPhone alone can't render the page correctly. From the screenshots she's sent me, it looks as though it's ignoring the media queries altogether, and rendering SVGs at whatever size it likes. This is not the case on other iPhone 4S' running the same version of iOS.

I initially thought that she may have been on a really old version of iOS that didn't interpret the media query width property correctly, instead using device-width, due to the 'new' Retina display, that an account version of iOS may not use correctly. However that doesn't explain the SVG behaviour, and the glaring hole in my theory: she's using an identical version of iOS to me.

Browserstack comes up empty, as everything renders fine there.

Thoughts?