If there isn't any way to just keep this other girl from playing with your daughter, then you will have to be more proactive.
Either way, go see the other girl's mom, RIGHT AWAY. Tell her exactly what happened and that you do not feel it is appropriate in any way for her daughter to play with yours in that way. Try not to be 'accusing' - to a certain extent, this older girl is at that age where girls like to start playing 'kissing' ect (although using the word 'sex' at all is really icky IMO, even if the 8 year old doesn't know what it means!) but it maybe something she picked up at school that her mom doesn't know about.
Hopefully this other mom will be reasonable and sit her daughter right down and have a talk with her about what is ok and not ok 'play'. I would also talk to her about the pushing, etc. and just say that maybe her daughter 'doesn't know her own strength' but that since your little girl is so much younger, you are concerned.
Try to set your daughter up to play with other kids her own age and encourage this other mother to do that same. A lot will depend on how the other mom handles this, but in the end, you may have to just tell her and her daughter that they aren't welcome to play with your little girl anymore because the other child is playing too rough.
If the other mom refuses to deal with the situation, then I would talk to some other moms in your building ( I lived in apartments for years, both single and with my son, so I know this can be a challenge, even in a friendly building!)
Talk to the mothers of kids the same age as your daughter and explain your concern to them and ask if they have had any similar issues. At the end of the day, the more grown-ups monitoring the situation the better. No one wants to just be a gossip or be mean, but if the mom of this 8 year old won't direct her behavior appropriately, other moms might have to step in. Sometimes it takes a village, lol.
Anyway, if you can make sure your daughter is with kids her own age and other moms are watching out, there is less likely to be trouble with the older girl. Hopefully too, even if her mom doesn't take YOU seriously, she will not want to look bad in front of all the moms in the building.
Good luck!