D.C.
This is going to sound really harsh but you have to just say no. I understand she works and its awesome that you are willing to keep the baby for her while she's at work but she's abusing the privalidge. I find it hard to believe that all the time your granddaughter spends with you her mom is at work. You need to set ground rules like you have to be here to pick up your kid at this time. I understand that she's young and she probably feels like she still has a life to live but she's a mother now and that has to come first before anything else. Its not that her child wont know who she is, the baby will know that that is her mom, the problem is when she gets older (3-4) and it's easier to care for her, her mom is probably going to be more willing to have her but shes not going to listen to her becuase her mom isnt an authroity figure in her life, you guys are.
It's about time managemnet and being willing to make sacrifices. IF its her job thats keeping her during the week she has to be blunt and tell them that she has to go get her daughter, she can't stay. IF hse still wants to go out at night she should plan it for a night that the baby is with her daddy. If its a special event and it can't be reschduled then a sussgestion would be to make plans to meet after the baby is bed, like nine or ten oclock, that way she can be the one to put her baby to bed and then pick her up early the next morning so they still have the day together.
You have to talk to her about this now though becuase as the child gets older it's only going to get harder. Good luck.