I just saw that too   He also opened an external link at https://www.digitalocean.com/community/tutorials/initial-server-setup-with-ubuntu-16-04  -  I really feel for him though, that's a lot of money to lose.  If that was his address and pvt key that were put on slack his money would be gone eventually anyway. 

Don't ever click an external link - or post your pvt key.  The best thing to do is to post on the Slack and ask the thief to hand it back, but judging by his post he doesn't seem to be worried about money, so he stands zero chance of getting it back because the opportunist thief probably needs the money a lot more than him.