Parents have their own way of addressing their child's behavior, and though you may wish wholeheartedly that the other parent would rein in her child more firmly, it's not your call. That said, there are ways to protect your toddler from a rambunctious playmate. To start, resist any temptation to take out your frustration on your child's friend. She hasn't done anything wrong other than test boundaries, which is what toddlers do. No matter how annoyed you are with her mom or dad for not intervening, keep your irritation in check.
Toddlers are smarter than most adults think, and they have powerful radar for negative feelings.
If your child's friend picks up on your annoyance, she may act out even more. Monitor the two children closely as they play to pre-empt any problems. When you see conflict brewing, distract them by playing noisily with another toy or breaking into song. And when your toddler's friend does something nice, such as sharing a toy without a fight, tell her how much you appreciate it. Eventually, just like any other toddler, she'll begin to learn how to behave and get along with others.