I think it's just plain common sense to say that doing twice as much of something is harder work. I also think it's a little bit insulting to the achievements of the women that do exclusively breastfeed twins.
schmee bf twins isn't always harder than singletons. twins are more likely to be accompanied by complicating factors (be prem, born by section), but I got lucky and had my twins normally at 38 weeks. I have found ebf them easier than feeding ds1 (who also had formula). I have never needed someone to pass me a baby, I have woken less often than with ds1 who was a truly awful sleeper, and I suspect dt1 was better as a twin than she would have been as a single, because when she was small, sleepy and finding it hard to latch, dt2 did all the sucking needed and she was able to just wait for him to start the let down.
Actually the hardest thing I found was maintaining my confidence in the face of HCPs and friends who assumed I would not be able to bf twins or that I should be finding it incredibly difficult, and this is why I wanted to respond to your statement.
OP: I hope your friend gets the support she needs. I have found twins hard, but feeding them doesn't have to be, though it sounds like they're having a tough beginning.
Mars is quite able to stand up for herself, so I won't boost her ego any more by saying how fabulous I think she is