Parents have their own way of addressing their child's behavior, and though you may wish wholeheartedly that your friend would rein in her child more firmly, it's not your call. That said, there are ways you can protect your toddler from his rambunctious playmate. To start, resist any temptation to take out your frustrations on your child's friend. He hasn't done anything wrong other than test the boundaries, which is what toddlers do. No matter how annoyed you are with his mom or dad for not intervening, don't take your irritation out on him.
Kids are smarter than most adults think, and they have powerful radar for negative feelings.
If he picks up on your annoyance, he may act out even more. Instead, monitor the two children closely as they play to pre-empt any problems. When you see conflict brewing, distract them by playing loudly with another toy or breaking into song. Eventually, just like any other toddler, he'll begin to learn how to behave and get along with others in his playgroup. You may also want to suggest another location for the play date.