Why doesn't part of my trackpad work?

The left half of my girlfriend’s Mac’s trackpad has become weird.

It is a MacBook Pro 15” Retina display Mid 2014 2.5/2.8GHz(Couldn’t find which GHz)(Dual Graphics)

The center area doesn’t respond at all, and the left half can move the cursor smoothly vertical but it jumps at 2-3cm(on screen) intervals when I try to move it horizontally.

The problem occurred after my laptop had been out with her all day. It was a cold and rainy day but it didn’t seem to have leaked in. She can’t recall installing anything that could have caused the problem.

I have followed this tutorial but to no avail. Is this a common issue with MacBooks? I don’t have much experience troubleshooting and fixing them.

Thanks in advance,

Hugo.