Adam and Laird had already offered to raise Hannah's baby with her, but in the series finale, Marnie was the one to help Hannah raise her son on Girls. While this sounded like a preposterous plan, a five-month time jump showed that Marnie was still around and helping out with baby Grover (yup, that's Hannah's baby's name). Marnie didn't have much going on since she was living at her mom's house, but was it really the best plan for her to decide to become Hannah's coparent? Well, the cracks were more than starting to show after five months and Marnie had to call in reinforcements to help.
As Marnie vaped in the pediatrician's office as Grover got a checkup, it was obvious that Marnie's revelation at the pawn shop probably didn't have a long-lasting effect. Although Marnie and Hannah already had a volatile friendship, the strain of raising a child together was clearly taking its toll on these best friends. Hannah was unraveling as Marnie refused to stop singing Tracy Chapman's "Fast Car" (TBF, it is the quintessential car-driving song) and then when Marnie tried to advise Hannah on how to breastfeed, she was at her wit's end.
While Marnie was being kind of insufferable (but when is she not?), she was there to support Hannah. So Marnie made the right decision to call Hannah's mom in to help since it seemed that Hannah was more emotionally distressed than normal since Grover wasn't breastfeeding. This visit from Loreen not only provided a wakeup call to Hannah, it also provided one to Marnie. Loreen made Marnie realize that while it's good to be there for your friend, you can't completely sacrifice your own happiness for them.
As Marnie was seeking happiness from masturbating with men via video chat as Hannah denied her requests to go out, she realized Loreen was right. Although Marnie was still with Hannah by the series finale, it seemed like she would be moving on to find some purpose in her own life — and this separation after helping Hannah raise her son during a pivotal time will most likely be something that will keep their ridiculous, (but somehow sweetly devoted??), friendship alive throughout their lives.