Does this boy have any outside activities -- anything in his life other than school and his family setting? School clubs, a sport, church youth group, Boy Scouts, anything at all?
If not (and I suspect the answer is "not"?) then some outlet that is his and his alone --not shared with the siblings, not something he "has" to do but chooses to do -- would do him a huge amount of good. He needs time and space away from his parents and siblings and he needs to have something that expresses his interests and his goals -- not mom's, not dad's, not school's, but his own.
If the mom is open to it, you could suggest to her that she work with him to find a summer activity that he goes to -- outside the house, without mom. She really cannot hover like this over a kid his age, or he is going to rebel and rebel badly, very soon. If he likes computers, find a summer computer camp for a week or two (not every "camp" means all day all summer long!). If he likes the outdoors, find an outdoor camp. Sports, a sports camp. Music, a music camp. Camp also does not mean he has to go away, which it sounds like his mom would object to; there are tons of day camps. Older kids have a lot more options these days and there are things for his age group out there but mom will HAVE to work to find them and be supportive of his doing it, and yes, she will have to accept that he is not around to care for siblings if he's at at a computer camp or art class or climbing class every day for two weeks! He will be with other adults. Most camps do full background checks on adult leaders or teachers now. And a camp or class in summer could open her to the idea of his having more activities when school starts. He really needs an outlet!
If cost is an issue: Some camps are cheap if done through local parks and recreation departments of your county or city government. Some private ones have sliding scale fees if there is a real, documentable need for them. Maybe you can offer to pay for a class or camp or outing as an early birthday gift for him.
Meanwhile, yes, talk to him; perhaps downplay the "witty exchanges" since that may not be where his head is right now. Try to get him away from the siblings. I'd offer to hire him to mow the lawn and do other yard chores on a regular basis with a set fee, to give him some independence and money. Or give him books about something that interests him. Always ask him what he's doing, what interests him, if he saw the latest episode of his favorite show (if you watch it too, that's even better; more to talk about).
I want to say a huge THANK YOU to you, for caring about this boy. He sounds a bit lost among his siblings, and it sounds as if his mom does care but might not be seeing that he is at an age where he needs to have more independence and not be expected to care for the siblings nearly as much. Getting him into some activities outside the home would help him feel he's got an identity outside the family -- something a kid this age really, really craves.
If I'm wrong and he does have some outside interests -- does mom stick to him at all of them? That would be suffocating for a boy of 14.
By the way -- do you think your friend really does "take out her bad moods" on him? Do you see any evidence that she is more critical of him than of the younger kids, or expect maybe too much of him? His comment about that is rather telling -- it may have just been unfounded anger or it may have been true, if she expects him to be a babysitter but is herself always peering over his shoulder as if she can't trust him....I would not interfere based on that comment, but I would keep my ears open to see if she does seem to expect him to be Mr. Babysitter while at the same time treating him as if he's not responsible at all.