Would need to know more....
Is she working (apart from mothering work)?
Is she under a lot of stress (apart from the stress of ptl, that is)?
Is she really truly eating/hydrating excellently, and sleeping well, sleeping enough?
I believe that a lot of preterm labor is brought about by 'too much output for the amt of input'--so, if a woman has a job (or even if 'only mothering' is not getting sufficient rest/downtime), then she may simply be overworked, especially an issue in 3rd trimester. Burning too many calories outside of gestation, her body just doesn't have 'enough' to support the pregnancy AND all else; survival mode demands ending the pregnancy on behalf of her own life. Or if there a lot of stress going on--financial, housing, marital, from extended fam (even something like an ill/dying parent or other close fam member), is anything else really a 'problem' for mom, emotionally--excess stress also burns calories and otherwise uses up resources that then can't go to the baby/pregnancy....so it might *seem* like she has a good enough diet and so forth, but doesn't really for the 'amount of output'. When I see a mom with ptl issues, first thing I help them do is careful review of input and output, stressors and so forth--so often, a mom just does not realize the underlying 'deprivation' going on (relatively speaking), doesn't understand how stress manifests in physical systems and so forth (it's NOT just in one's 'head'). But when she does reduce stress and/or work responsibilities, and/or increases her input somehow, ptl settles right down--her body now has 'enough' to work with to carry pregnancy on successfully and peacefully.
And of course, some babies come early because there are underlying and unidentified issues/anomalies (not always obvious w/scans). While most of these babies are allowed to pass in 1st trimester, it can also happen far later, even up to term. Have known a few women who did everything to carry to term in the face of repetitve ptl episodes--only to birth a baby who was incompatible with life. This is unusual, but it does happen.
Anyway--again, I'd be starting with the basics: amt of input vs output, a careful review of just what is burning calories and otherwise making her body work 'too hard to also support pregnancy', including any/all stressors. Sometimes women just don't see how much work/stress is going on, and so cannot identify what they need. In this era we take pregnancy so much for granted, we do not honor for the intensive WORK of our bodies and souls that it is--and too many women (and their fams and care providers) simply do not see this simple input-output equation at all. It's not as if we can always make stressors go away (tho we CAN choose to work fewer hours and/or get more rest)--but we can set boundaries, make an effort to detach from stressors, get help, etc.