Translation:Both he and she like Vietnamese food.
I'm not wrong, though I accept your point that it's a Vietnamese course, not an English one, and it would be better to allow both 'like' and 'likes' as both answers show understanding of the intended meaning. I guess when I originally wrote this, my answer 'like' was marked incorrect, and it should not have been. This is not to say that anyone who put 'likes' should be marked incorrect. From an English language point of view, of course, 'likes' is incorrect, but it does show understanding of meaning, so could also be considered acceptable. My gripe was not so much with 'likes' being deemed acceptable, as with 'like' being deemed unacceptable. I'm not someone who would insist on only standard Englishes being acceptable, but I do object to (a) standard English usage being deemed unacceptable. On another note, I'd like to respectfully suggest you frame your arguments in terms of accepted usage - something you are an expert in - and don't try to argue about points of grammar. Your reference to '”likes” (singular)’ makes no sense, but when you talk about 'what people actually say', you are far more convincing.
Tips sheet reads: "Useful expression: cả + subject + đều + verb/adj which is similar to “both” in English. It illustrates both mentioned subjects do the same action or have the same characteristic."
Should update to read: Useful expression: cả + [(subject 1) và (subject 2)] + đều + verb/adj which is similar to “both” in English. It illustrates both mentioned subjects do the same action or have the same characteristic.