How great that you are able to support this mama!
Low milk transfer is far more likely than low supply (although primary low supply is of course possible). The kicker is that poor milk transfer becomes low supply, as the body isn't getting signals to make milk.
I'd double check latch and active feeding, talk her through typical feeding patterns in a day (ensure she can recognise hunger cues and is not waiting for baby to cry, for example, and is feeding on cue - also setting no time limits on a feeding), is keeping baby close to her day and night so she is able to respond to baby's hunger cues. Also talk through why she feels she had low supply; could it perhaps have been a growth spurt, or normal evening cluster feeding which had her worried?
Making sure she knows about signs of sufficient breastmilk intake might help boost her confidence (e.g. nappy output - although after a few weeks poop is no longer a reliable sign - but soaking wet nappies still are).
It's likely that her supply will already have taken a hit if she's been offering formula for more than a few days; Jack Newman has some good suggestions here: http://www.drjacknewman.com/help/Protocol%20to%20Increase%20Breastmilk%20Intake.asp .Lots of skin-to-skin, co-sleeping if mum is open to it and will be following safe bed-sharing guidelines, and nurse nurse nurse!
Finally, if latch/suck seem less than optimal, or if tongue-tie seems possible, a referral to a GOOD breastfeeding counsellor or an IBCLC may really help. Sometimes mums need help in accessing the support available.
Just some thoughts...all the best! Would love to hear how you, she and the babe get on.
xx