My absolute best friend, since childhood, lives out of state. She is a divorced parent and fulltime student, with NO family or support system so I try so hard to like her 7 year old son, but I just can't stand the kid most of the time. And that is crappy, because he hasn't had the parenting needed...so it's not really his fault. For many reasons, sometimes wonderful people aren't necessarily gifted at child raising.
But STILL....he was at my house for a week and in that short amount of time, my son started imitating him in wanting to snack all day and not eat real meals, kicking rocks, leaving his mess EVERYWHERE, and the most irritating of all: the cocky shrug. ACK! It took a couple weeks to get it all back to normal. I do try to make arrangements ahead of time for my husband to watch our son, and for a sitter we know to watch hers, so we can go to our favorite Greek restaurant or get some daquiris and TALK without interruption. (I thought once if I could just survive until 8pm he'd go to bed, but he doesn't have a bedtime!!!) Now we just do childless nights, and it's rare to see each other, so it's ok to do that. We talk on the phone daily though, leave short little emails, we know we care about each other but the truth is that time, distance, and the rigors of motherhood DO shorten our time with friends. It is what it is.
One thing I HAVE done that I've seen success: I went to a class at the library on "positive discipline" and told my friend ALLLL about it and how excited I am to practice different little scenarios with my son, and how they work. I've heard her on the phone doing the same things I've told her....sometimes, parents just need some help that would NOT put them on the defensive or hurt them. Another thing we do when she does come to visit (which helps SOME, sometimes) is this: my husband takes him aside and says "we are so happy to see you again, and you know our little guy thinks you are SO cool...he will copy everything you do, so remember that you want to be a good example with him around so he doesn't get hurt or in trouble". We lay out very specific rules, supernanny style, and give our daily "menu schedule" of what times meals and snacks are, and what he can expect. Then if he starts bugging us about wanting candy (or doesn't want to eat his meal) we'll just remind him about when he's able to eat again (or that he won't be eating again for this long...). We also do try to go to parks or walk the blocks, the best we've found is in the summertime: the local spraypark is fenced in and is entertainment in a pretty small area so he can get some energy out without making us crazy. McDonalds playland does the same thing, if you want to have girltime over cheeseburgers.
And finally, with my love and sympathy for my friend (and sympathy frankly, for the son too) in mind, we do make efforts to love on and give her son experiences. Kids NEED parenting, and they need attention. My husband takes him on "man day" where they go to the barber, learn to skip rocks, play lazer tag, stuff like that. It's what loyalty and love for your friend are about.