they have gone separately before, yes.

I can tell you that on many occasions using miles I have found 4 seats with ease.

I will also tell you that while the low and desirable Y rate is 60k miles pp, sometimes in higher season it's 75k. And since non stop is a factor there have been occasions when my family and I would pay it. But sometimes not everyone has to pay that much. Sometimes if you book in certain ways, it's still 60k for one or two people. In fact one neat little trick I discovered with DL sometimes goes like this:

Begin to select flights for one or two travelers and it could show up at the lower rate. Come back and try to do it for 3 or 4 and it suddenly wants 75k for everyone for the same flight! So what I have done is to go back and just set up the one or two on one rec loc and then set up the others on a different one. So now even if some have to pay 75k miles, not everybody does.

This gets a bit tricky when flying with children, as was a recent case where my wife took both kids and I stayed at home. We were able to get her ticket at 60k on a separate locator and then go back and set up tickets for the kids on their own. But because the airline wants them to be with adults, we had to not make them children in the system. A bit of a date change. Then a little phone call to the airline to fix a typo before ticketing, and all was well. Not sure if this would always work. And of course THIS is the kind of thing I do not think my friend would be at all inclined to dive into.