K.P.
First, congratulations to her for breastfeeding! :-)
Breastmilk is naturally laxative, so I doubt that there's much she can do as regards her diet to help, although possibly something like drinking plenty of water and making sure that she herself is eating enough fiber and other things that contribute to loose stools. It's probably the formula that's doing it, although it's possible the babies aren't actually constipated at all. It's not uncommon for babies to go even several days without pooping and without being "constipated." Although both of my (single) babies were completely breastfed, they had very different baby poop stages -- my older son went several times a day, while my younger son usually went several days between poops. It's only considered constipation if the stool is hard and/or the baby has to strain to poop, or seems uncomfortable and/or unable to poop.
If the formula is the problem, she may be able to switch brands, but hopefully some other mommies will be able to chime in, because I never needed formula. I don't know if she will physically have the time or energy to do this, but it's possible that she will be able to build up her milk supply so that she can exclusively breastfeed all the babies, by pumping her breasts (even if they're "empty") for several minutes after each feed. When my younger son was a newborn, I started pumping for a friend's adopted baby, and by doing pumping while nursing him in the morning and afterwards, gradually built up to pumping 25 oz per day (and could have done more, but my friend chose to go to strictly formula after the first 6 weeks). I'm telling you this to say that it takes time to build up extra supply -- at first, I could only pump maybe an additional 1-4 oz., then 4-8 oz., and so forth and so on. For those weeks while I was trying to build up my extra supply, I would sit and watch TV and pump for maybe 15 minutes while I was getting one little drop after another. Breastfeeding quantity really is about supply and demand, and by the breast pump "demanding" more milk, I was able to build up my supply. At first, I was sitting there for 15 minutes getting almost nothing, and then I started getting many respectable ounces during those 15 minutes, and continued my "dry pumping" time for several minutes after I stopped producing, so my pumping sessions would be 20, 25, 30 minutes long, and perhaps more [I usually didn't time them, but just watched TV while my baby was happy or asleep and my older son played or watched TV with me].
She may not be able to do this -- I've never had triplets or even twins, so I do not even pretend to know what it is like. However, if she has help with the babies, she may be able to build up her supply like this.