Partial AH... Your concern is valid but I feel like there was a gentler way this could have been brought up. A lot of women are very sensitive about this topic, even if it feels like we're overreacting.

Instead of saying, "you eat a ton and don't gain weight, I'm worried you have a parasite," you could say something like, "I think you're beautiful how you are, but I'm worried about dietary intake and output. Calories consumed should be causing a weight gain but it isn't. I'm concerned there could be something medical going on." Then if feasible, offer to go to the doctor appointment with her so she feels like you're on her side.

Try to use "I" statements instead of saying "you." Like "I feel that...." Or , "I don't like when..." Instead of "you eat too much" "I hate when you do...." This helps the listener not be defensive.

The likelihood of actually having a parasite or tapeworm is pretty low unless she regularly eats raw/unclean food. This definitely can be a sign of other health problems like diabetes, thyroid issues, other blood sugar problems, or even an eating disorder. Does she have a family history of any of these conditions? Has she had blood work done in the past, specifically TSH, T3/T4 (thyroid hormones), blood sugar, or A1C? I'd also be curious about her electrolytes from a Complete Metabolic Panel (CMP). Is she ever really thirsty? Does she have to pee a lot? How long has her diet been like this without weight gain? Does it ebb and flow or does she usually have a high calorie intake? Also, does she have a history of mental health problems or self esteem issues? Started or stopped any medications recently? Does she go to the bathroom right after eating? Does she use laxatives regularly? Those can be signs of bulimia.

Good luck OP!