It seems like DS1/DDIL1 are very dedicated, but one thing I haven't seen mentioned when coming up with these scenarios is making sure the rest of the family is on board with any plans. Are your friend, her DH, and the other couple equipped to manage DGS during a meltdown/shutdown? If not and it always has to be the parents, can the rest of the group to do their own thing while his parents are calming him down? Will DGS accept being watched by the other adults when his parents are having their one-on-one time with the other children? Will that one-on-one time cause problems because he's not included? Do DS2 and DDIL2 expect to have some time together doing things on their own? Will it cause resentment if the trip seems to revolve around DGS's needs?
Even though it sounds like the extended family is close, I think the adults need to be very open and honest with each other about plans and expectations. I'm very close with my siblings, but I don't live near them. If I didn't really experience these issues on a daily basis, it might throw me for a loop when everyone's in such close quarters, no matter how much I knew about the situation.