Ok, this really does sound more like a case of real life stepping on the game's toes.
The situation you describe in game seems perfectly acceptable to me. +4 to a DC isn't anywhere near unreasonable particularly considering the people involved. I agree that two commoners arguing in the street approached by a stranger wouldn't give a penalty, but two judgemental clerics with superiority complexes are going to look down on this scruffy unknown... unless he has some sort of official rank and thus a reason for poking his nose in. Heck, from the sound of things, the Bard was lucky that the two lawful clerics didn't know him and his lawless behaviour.
I can understand the player's frustration, however, from two angles. One, I've played a bard before. It's one of the most disempowering roles in the whole game, since quite alot of the time the best thing you can do is boost the rest of the party and let them do all the cool stuff. It also sounds like this guy's under alot of pressure and worry... his wife is going BLIND, for christ's sake! A little compassion, people, please!
On the other hand, the player hasn't done alot to help himself either. From the sounds of things, he hasn't maxed out his Diplomacy score. Welcome to 3E, if it's not maxed out you might as well not have it. And he is playing a Bard... just about the only thing that makes the class fun and he hasn't maxed it out.
I can understand that this player right now needs to feel some measure of power and control over his environment. The situation in game has made him feel disempowered, which he is getting enough of in real life at the moment. Not that you did anything wrong, the dice and his own choices conspired against him.
I've got a few idea here that might help the situation.
1. Explain patiently that you can't nerf the Cleric and remain fair but you are open to retrofitting his character so that it is better (not a bad idea with any Bard, it's NOT for people who want to be a hero, it's for people who want to support others).
2. Note: I'm also in a situation where everyone comes around to my house to play. I don't have 3 kids and cleaning the house is still a bitch. Playing at this friend's house is a good and neighbourly thing to do. Now, I'd suggest extending some of that neighbourlyness to THE DUDE WHO'S WIFE IS GOING BLIND. I suggest excusing him from driving and don't use his car. It's the decent and compassionate thing to do, the guy is driving all the time, busting his hump trying to care for his family. The least you can do, as a friend, is drive him for half an hour every now and again.
3. Throw in some encounters where the Bard can shine, maybe an item or two for him. I understand that the limitations presented to you by being at this point in the campaign but this is an extraordinary circumstance.
4. If he remains intractable, suggest that his limited time might be better spent taking care of himself and his family rather than playing a game. I'm honestly amazed at this guy's fortitude in not quitting the game with everything going on in his life. It would be a shame if he quit, if he does in fact enjoy the game so much that he chooses to spend his limited free time it might be one of his few outlets of release right now, which is what's making him inappropriately cranky.
5. Let him know that there are people on ENworld praying for him and his family, and give him our best wishes.