Does HE want to stay over at friend's? If he doesn't he won't care to stop.
Diapers at 6 unless he is special needs should be stopped, as should the BM's at night, he's way to old for those, deep sleeper or not...Dad needs to stop making excuses for him and expect him to at least poop before bed. Make it your son's responsibility to clean himself up in the morning, if you get him in underwear and he poops them teach him how to wash them out, and if he goes to Goodnights make it his job to strip, toss the poop in the toilet and flush it (because "that's where poop goes") toss the Goodnight, then clean himself, refuse the "greetings." Have him go to the bathroom after dinner, no TV, video games or toys until he poops, then bed. Also try to regulate his pooping schedule so he does it earlier in the day.
As far as bedwetting, if he still is at the age of 8 or 9 his pediatrician may do tests to determine if there's a medical condition causing it or not and you can go from there.