Also remember that our tongues are pretty good judges of what's good for us and what isn't. If it was poisonous odds are it would have tasted bad and Greer would have spit it out. If it was actually caustic he probably wouldn't have been able to pick it up.
Our tongues are good at that in our own biosphere due to many millennia of ancestors dying off whose tongue couldn't detect the problems. I'm not nearly as confident that we'd be able to do the same with alien plants.
(Nor that, of course, we'd be able to digest them or receive any food value from them, but that's a whole 'nother level of trouble.)
I wrote up a whole long spiel about this, and then deleted it because I figured no one would care that much. It involved anti-freeze fruit.
Taste is a good first indication, but not necessarily the best. Ethylene glycol is very poisonous; very little is required to kill a cat or dog, and not much more to kill a person. It's very sweet (sometimes decribed as sickly sweet) to the tongue.
The best thing that Greer, or anyone else, has going for them is size. It will take a lot more toxin to flat out kill a human than a cat or dog. Second would be a purging response, in which dehydration would be the major medical problem. Realistically, Greer might get sick but probably would not die.
While alien food may or may not be digestible, there is generally a limited amount of possibilities that make it more likely to be able to be.
First and foremost - any planet that is hospitable to humans will certainly use a carbon-backbone for life. Most alternative systems only make sense in a completely inhospitable environment. Silicon-backbone stuff would be immediately recognizable.
We won't assume that DNA, amino acids, and proteins necessarily exist. However, simple sugars (pentoses and hexoses) almost certainly would, as would basic fats and fatty acids, given a carbon backbone. Polymerization of simple sugars is highly likely as well, giving starch- and cellulose-like substances. All of those are highly digestible through relatively non-specific systems. Of course, you could argue (quite rightly) they wouldn't necessarily use oxygen for electron transfer. However, alternatives are all fairly easily detectable by humans senses. Nitrogen-variants would smell fishy or astringent, due to ammonia and various amines as by-products of dehydration reactions. Sulfur-variants would similarly smell foul due to sulfides. In contrast, oxygen dehydration produces water.
Really, it's not energy you need to worry about though. All the best toxins interfere directly with essential systems, especially the electron transport chain (makes energy). However, if we don't have Earth-like amino-acid proteins, the chance for interactions there is relatively minimal. Most of it would pass through relatively undigested. There is always a slight chance that something might get broken down into some analogue that clogs a system. The best thing to do in that case is to overwhelm the body with stuff that
will get broken down; it's probably more specific than some random plant on a planet. Given you don't know what it is, you probably force-feed the patient anything you have from Earth to cover all the bases.
