You have the right to set limits and rules within your own home and to ask that people who come into your home abide by them.
I have handled this kind of situation using the following steps:
1. Determine what 3 rules are THE most important for the child to follow right now. Try to make them specific. For example, instead of "act nice" you could say "do not touch another person except in a gentle manner."
2. When the child arrives at the home, corral him and your son. Tell them that there are 3 rules that must be followed. Tell them the rules -- write them down on a posterboard or wipeboard if they read or draw pictures if they don't. Have the kids repeat the rules. Ask them if they understand the rules. Then check that understanding by giving examples.
Also be sure to spell out the consequences of refusing to follow the rules. For example, say, "I know that you are going to walk in the house. However, if you forget, then I'm going to put a check on this paper. Five checks and I'll have to call your mom and send you home."
I found it most helpful to phrase the rules like this: "The house rule is that we touch people gently." "The rules in this house say that we walk when inside." Putting the rule 'on the house', so to speak, lessens grounds for arguing with you.
Positively phrased rules work better: Use polite language instead of No potty mouth.
3. Try to catch the children doing the right thing and reward verball: "Wow! Will, I really liked the way that you gently moved Mark out of the way instead of pushing him over." Be generous with your praise -- and specific. Reward approximations of the desired behavior: "Cool Will. That time you skipped instead of running. That's almost a walk!"
4. Be prepared for a certain amount of testing. You MUST follow through on the consequence or this will be ineffective. Five checks means five checks -- send him home. If five checks becomes six becomes seven, the child will disregard your rules.
5. Explain to the other mom what you are doing. Try to do it without blaming her child. For example, say, "Susan, we are trying some new rules at our house. Everyone must touch each other gently, walk when inside the house and use polite language. I need your help to teach these rules to the kids. That may mean that I have to send Will home earlier than planned if he or my son aren't able to follow the rules."
It's hard for a parent to argue with global changes to the way your house is run. By stating a desired outcome (walking in the house) instead of issuing blame (Will constantly RUNS in my house!), the other mother has less need to defend her or her child. She is then more able to do her part in achieving the desired house rules.
I hope this works. I have one formerly-unruly child that has been hanging out at our house for over five years now. It took about 6 weeks of constant review and consistent appliation but now he fits in as well as the other kids that come over. In fact, I've even heard him tell a new friend, "We all walk inside this house. It's the rule."