(Assuming you are referring to "plain" beers, not beers with weird ingredients)
The only allergen that is present in normal beers is malt that contains gluten. Gluten intolerant people can therefor react to beer very aggressively.
Next, I would guess the alcohol. Some people have violent reactions to any form of alcohol.
Then, maybe the bitterness or just the flavour. The people may be sensitive to bitterness, or just really do not like the taste of (that) beer.
I have had beers that taste like baby vomit. It is VERY hard to not gag when you taste that!
You have to determine why the person reacts that way. If it is the malt, then get them a gluten free beer, if it is alcohol, then try an alcohol free beer, or just a cold drink, if it is the bitterness, then get a beer that is less bitter, if it is the flavour, then get a beer with a different flavour (fruit beer or a different style of beer that has other prominent flavours).