J.S.
I have the same problem with my niece's kids. They were over at Christmas and the 2 1/2 year old was just a nightmare. Granted, he was doing typical toddler behavior, but my home isn't child-proofed anymore and his parents knew it - we discussed it with them before they came over. His parents didn't care. They spent most of the time either smoking in their car or texting their friends on their cell phones. Fortunately, said nephew's grandma (my SIL), volunteered to watch him. My other SIL and my oldest girl kept an eye on him too. I was too busy being a hostess to watch him. Another nephew threw food at his brother. I witnessed that and reprimanded him right then and there. "There is no throwing of food in my house. I don't care what name your brother calls you." His father (my BIL) heard me and took the kid aside and echoed what I said. It was nice to have the back-up.
I'm a pretty big proponent of "my house/my rules" and "kids need boundaries." Actually, most kids are better behaved when they are at my house. The parents are suprised to hear that their kids were good or were no problem.
I don't buy the excuse of "boys will be boys" either. All children need to learn and abide by the rules - boys don't get a free pass just because they are boys. They have to learn to sit still and listen too. Parenting any child can be tiring - so don't give me that BS. Good parents are consistent - even when they are tired.
Lastly, when my kids were little and we were invited to someone's house, I watched them. I didn't expect my host/hostess to run after my kids! Sure, I couldn't necessarily relax and enjoy the time - I'd have all these partial conversations. My husband and I would take turns. I always thought that was part of being a good guest - I would keep an eye on my own kids. When we wanted to go out and have a relaxing time, we hired a sitter.
In your case, I would keep an eye on the kid and if he did anything dangerous or something that could harm someone else, I would step in and tell him no. Or if he was screaming. I would pick him up and put him on his mother's lap with a not so subtle, "Watch your kid!"
Good luck!