If she's not too tired, she will birth the placenta. Especially if there are still more kittens. If this is the last one, it is likely to be awhile.
My last foster went nearly an hour with the placenta unbirthed and I eventually had to do it for her.
To do this: GENTLY
tug on the cord until it is out. I stress - do this GENTLY as you dont want it to rip or cause a the kitten to bleed into it's cord.
I hold onto the cord in two places, one on the kitten side and tug on the mom's side, this way I am sure there is no pressure on the kittens tummy.
After it is born, either cut the cord yourself, or show it to the mom and see if she'll take care of it herself.
And Congrats on the two new kitties! Cant wait to see pics