MacBook Pro Mid-2012 starts but does not respond anymore

My gf has a severe issue with her MBP Mid-2012 (serial number is C1MK9X6ZDTY3; it is a MacBook Pro 13-Inch "Core i5" 2.5 Mid-2012). A few months ago I replaced the internal HD with an SSD. Everything worked fine. A new SATA cabled was also put in place.

Recently however the laptop had some issues. First, when pressing a certain area on the case, it would reboot. However, after rebooting I could work with it again as if nothing has happend. A few days later, it would again shut down, but then make 3 beeps while attempting to reboot. As several sources online state, this means faulty RAM. Intuitively I just tried to reboot again and the MBP worked just fine. My gf told me that there were cases where the diplays showed some glitches. Right now the situation got worse and the laptop cannot be used anymore. It boots, but then the "spinning coloured ball" occurrs. The laptop responds extremely slow. Finder cannot be opened, etc.

One more thing: She just told me that a few weeks ago she switched on the laptop with the power cable connected because the battery was empty. She then noticed some sort of "short circuit" when connected the cable. There was a tiny flash and it was as if something "burned". The funny thing is just that I would expect that the laptop immediately stops working. However it worked fine afterwards, but (obviously) slowly deteriorated as described above.

Other things I have tried:

• I run Apple diagnostics. It said there could be an issue with the battery. I then did it again with power cable attached and it said there are no problems.
• I removed both memory sticks (one by one) to ensure that none of them are faulty. With only one of them in use, the system behaved in the same erroneous way.

So what whould I do now? Can I do anything at all or should I go to a technician? Which faulty hardware could be responsible for the described behavior?

UPDATE: A short update: I bought a Samsung 850 EVO 500GB (see here: https://www.amazon.de/Samsung-MZ-75E250B-EU-interne-schwarz/dp/B00P73B1E4?th=1). Actually, I just remembered that I did not buy the SSD. The SSD is already a bit older, but I bought a new SATA cable, because the SSD wouldn't work internally. So I bought this cable: https://www.amazon.de/ireplacement-A1278-Drive-Macbook-821-1480/dp/B075PZ3PK5?th=1 With this cable I could use the SSD internally and everything worked fine.

UPDATE: I identified the root cause. See this thread now please: Extreme slowing down of MacBook Pro due to pkd process

Solution 1:

One thing that does happen at times with MacBooks of this kind (own one myself and had it happen to me) is that batteries can over time "inflate" and slowly but surely push themselves against something — in this case it would be weird if the something was the mainboard since it's not located under the battery but rather beneath it). But maybe you could take it apart and see if the battery looks odd or even a little blown-up and ask your friend if she ever replaced it before..?

Some basic steps to perform:

• To rule out possible battery faults, you could take the battery out, then try turning it on with the power cable attached
• Give the obvious "mac-things" a try such as SMC reset, PRAM/NVRAM reset etc.
• Take out the SSD and attach it to another computer to see if you can start it up from there