What is the best way to help with other's Children

I have a friend and him and his wife are very busy people, they get praise for all the things they do, however, they have three kids who are bossy and picky, and do not behave well in others homes, what would be the best way to handle this?

That is not an uncommon situation with kids whose parents are away at work a lot.  You might try simply listening to them - engage them in a conversation about something they like.  If they're not too far gone, this often helps to curb some of the fussiness - they want attention.  If they're really over the edge, if you have to deal with them, simply set some firm rules and stick to them.

I think that when they're in my home, they have to abide by my rules, so I explain "this is the way we do it here," and keep an eye on them to make sure they comply.  With children who have very different expectations in their own homes, this can take a while with a lot of gentle correction, but generally I've found the children want to get approval and so try to learn and follow my rules.  It also helps to explain WHY we do things this way in this house, so they understand why I'm asking for this behavior or banning another one.