After the 'Darker Shades' achievement, head back down to the ground floor bar and talk once again to the bartender, Bobby Bao. Accept his mission to find a female called Jaya to get his money he's owed. You'll be tasked with finding her hiding location via 3 hacking devices on some satellites. Once you have her location head to it and confront her.
There's now two choices you can make here and both will unlock the 'Bar Tab' achievement but only one will unlock the 'Guardian Angel' one. So we'll do the two birds with one stone option, but for this you'll need to have $5,000 on you. If you don't feel like losing out on all that cash make a save before you talk to Jaya.
To get the 'Bar Tab' and 'Guardian Angel' achievements both in one go, you'll need to confront Jaya and understand what her problem is. In other words, she's not going to pay back the debt she owe's Booby. So head back to him and offer to pay Jaya's debt for her as long as he leaves her alone afterwards. He agree's, so simply give him the cash and both achievements will unlock.
If you want to keep your $5,000, reload your last save after the above achievements, and don't agree with Jaya and do a none lethal takedown on her. Steal back the debt payment - which is an Augmentation - and return it to Bobby.