Comfortable, when it describes objects or places, tends to refer to physical comfort (softness, temperature, ergonomics), and this sense may be jarring when applied to food. I will quote most of the Merriam-Webster Dictionary entry for comfortable (adj.):
1a : affording or enjoying contentment and security // a comfortable income
b : affording or enjoying physical comfort // a comfortable chair // was too comfortable to move
2a : free from vexation or doubt // comfortable assumptions // Lamb was comfortable in his ignorance of what he did not choose to know.— James Mason Brown
b : free from stress or tension a comfortable routine // stayed at a comfortable distance from the crowd
Your friend used comfortable in sense 1a: the food affords contentment and security, or a kind of abstract comfort. In comparing it to the couch is comfortable, you insist that the food example should be read through the lens of sense 1b, physical comfort. Does usage follow your intuition?
In a corpus search, a collocation search for "(something) (something) is comfortable" will tend to turn up objects of physical comfort. For the Corpus of Contemporary American English, I've categorized the results with 2 or more entries, excluding names. There are:
Words related to people
Words related to spaces
- itself ("the bridge itself," "the cockpit itself")
Words related to objects
So in non-unique results, people, spaces, and physical objects dominate. Physical comfort seems the predominant usage when applied to objects or spaces, rather than abstract security or comfort.
Does "food is comfortable" appear? Not in that corpus or in the larger News on the Web Corpus. It does appear on the web - about 23 results on Google for "food is comfortable" compared to about 439,000 for "shirt is comfortable." Based on the corpus results and the dictionary, I suggest people will use comfortable far more often to refer to physical than abstract comfort, and food is in the odd situation of being physical but giving abstract comfort. So you can say "the food is comfortable," but it is rare and potentially confusing as people try to decide precisely how you sit on or wear your food.