Aunt's MacBook Pro
At first the issue was with booting up. The MBP would not get past the progress bar screen no matter how long we had waited. Using Command+Option+R+P didn't help. What finally helped was booting it into safe mode. Every boot up after that would work. Yay!
Once it was logged in, performance was abysmal. It would take a good minute or so for anything to happen. I checked the battery and it wasn't accepting a charge at all, and it had only gone through 220 cycles! Since it's a 2011 model Apple doesn't carry the hardware to repair this anymore, so this was my only course of action.
The repair went quickly as easily, like pulling off a band-aid. I had actually live-streamed the whole process on Twitch.tv.
Despite what the guide says, when removing the battery, I actually had to use a Y0 screwdriver and not a Y1. I bought the iFixit Pro Kit for this repair and I'm sure I'll get much more use out of it.