I feel your pain - been there and done that!
From my side this is what we learnt...
1) it's often not only one boys fault. I assumed that my boy was being easily led and it wasn't until I saw irrefutable evidence that mine son was, if not the ringleader, then at the minimum quite happy to tag along that I realised there ere quite a few making it happen
2) agree about different parenting styles - nightmare!
3) I found that the best way to deal with it was to welcome the other child over and do the things they loved (activities/meals etc) that meant that I had some weight when I could say to him "We don't always behave like this, do we?" To be clear I was parenting the other child, just making sure he knew I didn't approve then I would turn off the playdate tap for a week or so and then welcome him back with open arms. Trust me, if he gets his favourite food/TV show/activity at your house and he's bright he'll quickly realise that actions have consequences
4) it is very easy to get into a stress about this and what I found worked was to tell my child to behave, to be kind but have zero tolerance for bad behaviour. So if they start to mimic what they've seen you come down on it hard, they often come back with a "but XXXX does it all the time!" and my response is "and you're not X, so we don't do it". Again kids are pretty good at understanding that not everyone behaves the same. They try to take advantage of bad behaviour as something they can copy and blame but they're not stupid...
5) it will blow over. It stressful and awful but you'll be laughing in a few years about this. Just ensure standards at home are kept up and all will be well.
Now when they come back from a friends "dirty" house with nits every playdate - THATS a dilemna!!!