friend getting very sick from eating onions, anything that would help?!

Onion is from the Alliaceae family which also includes garlic, shallots, chives etc. it could be a food intolerance which means that your intestines do not make the enzymes to break down the specific food, best way is to avoid the food as best you can, but another way is to drink lemonade or mineral water and chew mint gum as the sick feeling is usually caused by the part of the onion that makes it smell so getting it out of your mouth can help, the actual stomach aching is the intolerance itself which you can get tablets for. Also intolerance is a mild for of an Allergy although they are not in the same category i would just say you are allergic to make sure they leave it off.

Allergic to garlic, intolerant to onion, shallots, chives, ginger, cinnamon and bananas.

Dairy might neutralize it, but I have the same sensitivity and dairy doesn't work for me. The discomfort doesn't last long for me but he may be a lot more sensitive.

Your friend is going to have to start telling them at restaurants that he is allergic to onions...it's not really an allergy but here's why he needs to do this: When you tell them simply not to put them on, the assumption is that you just don't like them, but if you tell them you're allergic, they take that much more seriously.

If he has an allergy to onions he will need medical treatment,although it would probably be best to vomit.
It seems to me he is the stupid one for not checking there was no onions, more so because he even ate the Sub, and carelessness on the part of Subway.

Tell him to drink a lot of water and eat meals packed with fibre!

maybe a glass of milk or some soda like ginger ale or 7up.