Hi Tim-
Perhaps by now the situation has already been rectified, but as a mama of a similarly reflux afflicted daughter (when she was an infant), I feel your friends' concern. I guess I have more questions for you....
You mention that "she falls more and more behind her the average weight for her age"...
Are you saying that she does not have a consistent growth curve? Or that she is actually losing weight (when not ill)? Our daughter was born on the 3% (far below "average") weight percentile curve. And while with each weight check she seemed to be gaining SO much less than her twin brother, and seemed SO small compared to more "averagely-sized" babies her age, she was rock-solid "consistent" on that 3% curve...the reason our doctors (perhaps like your friends' doctors) also entreated us not to worry. Our daughter was placed on the prescription Zantac for her reflux early on...as a small baby, the excessive spitting up (and gas/acid discomfort and "exercise'/calorie-burning associated with that) was mandating intervention. (We were breastfeeding, and I made all sorts of attempts to alter my diet, all to no avail...she was just refluxy!)
Age one does seem a bit "late" to still be suffering from severe reflux...I trust the doctors are monitoring that. Of course at age one, she may be eating "more than she can hold" as well!
One thing that made me feel SOOO much better was when I told my pediatrician that I was confident everything I was feeding was coming back up, she suggested that I do a genuine guesstimate of the content (ounce-wise) that our Sarah had consumed in the meal. And then to fill a baby bottle or measuring cup with that amount of water. After a big spit up, I'd then go to that measuring cup and "dump" on a cloth/towel to (I know this sounds gross) replicate the diameter of the spit-up puddle. I was continually amazed that what SEEMED like it must be an entire feeding as a spit-up puddle in reality was often a MAXIMUM of a 1/3-1/2 in water spill of what the estimated meal was. Sounds archaic and ridiculous, but it was SO very reassuring.
More reassurance, our daughter was an impish 16 pounds at age one. (not much more than 7kg.) When after a tummy bug she dropped to the 1% growth curve, THEN the docs began watching her like hawks for a awhile.
Ultimately, if your friends do sincerely feel "something is not right" they need to relentlessly continue to visit doctors until someone can explain to their satisfaction why she is either so tiny, or what might genuinely be a cause for concern, losing weight. I am NOT a doctor, but I so remember having similar concerns...as a parent, you're the biggest advocate. Keep asking until you get answers. It's the doctors' job...no need to feel they are "inconveniencing" the docs!
Is their daughter otherwise happy? Healthy (beyond the spit ups)? That was the "alarm" the doctors always set for us. Eventually, we just got used to the fact that in every 100 (upon which a percentile/percentage chart must be based) someone will always be the "third from the smallest." We just happened to get that girl! :) Now at age 6 she is still small for her age --- 38 pounds ---her 4 year old cousin who's "average" weighs more, but is our Sarah is moving "up" percentage-wise.
Hoping all is well (and I am thinking it likely is...) with your friends' daughter....as a mom of a former refluxy and still teensy girl, I remember that anxiety well.
You are a kind friend to want to help!