I have a friend that doesn't discipline at all either. Granted there is no pushing involved but it is tough.
When a child is being pushed or upset by another, I step in. Either nicely say something to the other child or the to the mom, if she doesn nothing then I would speak up and say "I am sorry, but I do not want my daughter to pick this stuff up as being okay to do, we need to go now" and that is it. I cannot believe she didn't say anything but you know I never feel badly for standing up for my kids at all!! I even do it with strangers kids at a park, find the mom and explain, well your son pushed my daughter down and I don't appreciate that but he isn't mine to discipline! If she gets angry or upset with you, that is just the risk you need to take.
I have a friend I posted about weeks ago that lets her daughter rule the roost then calls me complaining she is out of control, the girl laughs at her mom and doesn't listen, has HORRIFIC table manners, won't eat barely anything other then her favorites and her mom is the cause, I don't blame the little girl. But because I am not the parent, I just have to start kind of cutting her calls short as I don't want to hear her rant for hours about how out of control her daughter is when she doesn't want to do anything about it! :) Make sense? So I just kind of side step the conversation and we have avoided them coming over because my kids wonder why the mom doesn't do anything. All she wants to do is complain about it and how wiped out she is and all of that and the only way I can save the freindship is not say anything right now!
I say defend your daughter, even if the mom says nothing look at her son and say "that wasn't very nice", it is not your job to enforce punishment but you certainly can speak up, seeing you do this will empower your daughter to stand up for herself! Kids have issues with sharing at that age, but at the same time it is your job to make sure your daughter understands why it is important and make it a positive. Praise her when she does share maybe loudly when you are in the prescence of your friend. I think if you are good friends you can maybe side talk with her about it nicely, otherwise maybe forego playdates until she gets a gripe to control her kids better. Sorry not much help, it is hard to bite your tongue, just have to decide if you think it will help or not to say anything.