Lets back up here a bit...
Your girlfriends MacBook Pro is a retina model (blade SSD) or an older non-retina model (2.5" SATA)? Can you clarify what you have here.
You stated you bought a SATA to USB adapter which would imply you have a older non-retina system but you've marked your system as a retina model. If you indeed have a retina model then you'll need this adapter: OWC Envoy Pro Portable, Bus-Powered USB 3.0 Enclosure - For Apple Flash SSDs (June 2013-Current Mac Models)
As for the power issue your MacBook logic board is likely damaged which could have been from the SSD or some other device. To check things I would get this little tool: USB 3-in-1 Voltage/Current/Capacity Meter. You also may want to get a powered USB hub as it may allow you a means to over come the power issue using it instead.