Dear Jaye,
In my experience, milk supply being "suddenly gone" at 4-5 months is usually a direct result of scheduled feeds.  At least in my area, many peds start pushing sleep training etc. around 4 months.  It doesn't exactly sound like that's what's going on with this particular mother since she did pre and post feed weights all night.  You said no meds, but this is also a typical time to start hormonal birth control and unfortunately some mothers don't consider these to be meds when asked.  The other thing that occurred to me is that a mom with good supply or even oversupply due to the initial surge of lactation hormones can gradually downregulate to crisis level if baby is unable to remove milk sufficiently due to anatomical issues such as tongue-tie, submucosal cleft or sensory integration issues.  
Finally, the thing I always think of is a thyroid storm or possible development of hypothyroidism postpartum.  Diana West also mentions anemia as a possible cause of low milk supply.  Suggestions for a blood panel (if you can work up the courage given the new Scope of Practice!) can inlude:
Thyroid (TSH/ free T3/T4)
Fasting Glucose
Testosterone
Estrogen
HCG
Progesterone
Prolactin 
Iron
Ideally of course, the best thing to do would be to determine whether it's the baby's issue or the moms which is causing the low milk supply in the first place. 

Sincerely, Liz Howell, One of the usually silent 3000, LLLL Queens, NY (fresh from 2 sessions on low milk supply by Diana West at the NYE area conference!)

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