It's my girlfriend's tablet, my son had it when screen blacked

Check for the KNOX flag and you’ll know if your son messed with it in ways that may be hard to recover from. You can check for the 0x1 flag (tampered) by going into OS Download mode by using these instructions. If your son DID root the tablet, you’re in the dog house if she uses it for work and has to have the 0x0 flag - it erases the encryption key used by the KNOX workspace and disables the feature permanently (eFuse). In the past it could be reset with JTAG but most people on this site would make things worse if they did try the JTAG reset. In addition to that Triangle Away hasn't worked since Android 4.3 bakes it in now from the Samsung factory.

The other indicator (easier for end users) is if you get the custom triangle at boot - this WILL get you into trouble since she will see it as it comes up at boot persistently. If you're worried she will not take it well, my best advice is bribery unless it's company owned. Pay her off to cover the cost to put the fire out early. If it's a personal device make it vanish and call it a wash once she does what she needs to. I mean to an extent you can technically argue you bought it out and say you had a right to do what you wanted, but be careful with that. If it has the 0x1 flag you're basically better off paying her off since it's days of working in a office are dead. Been there (not with girlfriends, thankfully) in the past. I usually pay them off, keep the device until they forget I broke it and make it disappear or repair it; depends. My general approach is rather than find out they unlocked is I will find a factory unlocked SKU, and be clear I did so so I don't need to worry about unlocking it. Not always possible but generally works out these days.

If it’s just black, you need to do additional troubleshooting. If it’s a cracked screen, you can find a Google FRP donor to get the part from and fix the tablet that way.