I can see where Vit K may cause a slight increase in jaundice--ANY medication given to the baby that is processed by the liver is going to take a bit of time away from the liver and may make a slight difference in bilirubin processing.
However, I can tell you that in my several years of working in an out of hospital birthing environment where many, many people declined the vit K, I saw no evidence that vit K babies had more problems with jaundice than vit K-free babies. My thoughts are totally anecdotal and not scientific. They're just my personal observations.
I would think that the prolonged crowning and shoulder dystocia can cause jaundice, particularly with bruising. And sometimes, babies are just jaundiced. What is the gestational age of this infant? Any other medications given to mother or baby during labor or immediately afterwards? How long did it take her milk to come in? Is baby a good nurser? How long did she push? Was her water intact during pushing? Did baby have a lot of molding/a big ol' conehead? What are the parents' blood types and is there a possibility of incompatibility?
All of these factors can contribute to jaundice. If her mw is a typically non-interventive or at least a thoughtfully interventive type, and she recommended vit K, I'd be inclined to trust her. Not that you should never question or evaluate your mw's performance--I'm just saying that none of us were there and therefore cannot understand the thought process behind the mw recommending the vit K even in the event they were planning on declining; none of us are there to evaluate the infant and the mother to determine if any of these other factors are an issue.