I guess this is really a question for our resident gourmet Mangetout but I’ll go on anyway.
I have a friend who eats onions in much the same way as most of us eat apples.
He skins an onion and scoffs it down, in large bites.
Now I can’t see that this is good for him and I’m hoping Mangetout or anyone else can confirm or deny
There are varieties of onion that lose nearly all of their pungency after a month or so of storage - they’re marketed as being able to be eaten like apples.
If this is not the explanation for your friend’s onion-munching habit, then yeah, maybe he just likes raw onions. In moderation, not likely to do him a lot of harm apart from watery eyes, stinky breath and perhaps indigestion.
Personally, I love raw onion (with grated cheese, in sandwiches is my favourite), but it doesn’t like me. I get hangover-like symptoms within about ten minutes of eating them raw.
Why would it be bad for him?
The onions he likes are the big fuck off Spanish ones…hot as hell.
As to his breath, it’s odd but only occasionally have I detected a whiff, most times he’s OK, not at all pongy.
I also like raw onion especially with Double Gloucester or Red Leicester.
Now I could kill for a cheese sarnie 
I usually find the really big Spanish onions to be quite mild and watery - those little hard ones you get in the Tesco ‘value’ bags are the ones to watch.
The pungent compounds in onions are irritants, so I suppose it’s possible he could suffer some kind of… well… irritation from eating them raw in large quantities, but maybe it’s something you can build up tolerance to - like hot peppers.
Right, that’s answered that.
Rather than start another thread: Can I use the vinegar left over after using all the pickles in the jar, to pickle raw onions?
And if so what onion would you recommend?
I wouldn’t re-use it, as it may have become diluted by juices from whatever was pickled in it originally. Vinegar is so incredibly cheap, it’s just not worth the bother.
What variety? Dunno - if you’re buying the onions for pickling, you’re not likely to have much of a choice. You could pickle shallots - that might be interesting.
Well, in some species onions (in large amount) are toxic. In the dog and cat, for example, onions can cause a Heinz body anemia.
Onion Toxicty
And of course too much of anything is bad for humans too - that’s why it’s called ‘too much’.
Davidson makes no mention of onion toxicity in humans. He does mention that onions were eaten raw, and in large amounts throughout the classical world. The literature of onions seems to be especially rich from old Egypt.
So people have been popping the things down the pie hole raw since earliest times with no ill effects.
(The onion and cheese plate at McSorely’s near the Village? To die for.)
Can you explain this to the fuzzy black land shark who lives in my house please? When I go into the kitchen to cook, he doesn’t bother me until I start cutting my onions or shallots. Then he immediately runs into the room and begins the squealing ankleweave. I tell him he doesn’t get any, but he keeps up the racket until I let him sniff the knife; he puts his ears back in displeasure at the aroma, and leaves.
Stupid cat.
Except chowder of course.
You can never have enough chowder 
On the subject onions I did hear once that they were the most consumed veg in the world.
I would have thought it was spuds
I don’t do it on a regular basis but I sometimes eat onions in that manner. They’re delicious. No ill effects that I recall.
Persian/Armenians think nothing of eating very strong raw onions as an appetizer. They usually give you them along with the bread course. They have a salty/paprika mixture that you shake on it that makes it a little easier to take.
I seem to recall many narratives of days gone by in which rough yet honest plowmen walking along the toll road eating their lunch of a hunk of bread and an onion.
There is a scene in one of the plays about Thomas A Becket of a young novice monk eating a Hastings onion that way. That would have been about the the 1160’s, so this has been common for quite a while.
My ex father-in-law used to eat a raw onion every day. He said it was to prevent colds and flu. In all the time I knew him he never had either.
In the Classical world?
Your typical Spanish “ensalada mixta” usually includes some onion, I didn’t know I lived in the Classical world! I don’t know many people who’ll just munch one, but I do know more people who’ll eat some raw onion with their lettuce than people who’ll eat some boiled onion with their cooked veggies.
Mileage, as usual, varies.
now I want me some nice salad…
I have pickled Shallots before. They are quite tasty. Slice them thin with a mandolin, and make a pickling liquid out of rice wine vinegar, mirin, a splash of sherry vinegar, sugar, and a little grenadine to give the shallot slivers a nice pink color. They go well with seared tuna and unagi sauce.