Not personally (knock on wood - So far as I'm aware, my only allergies involve mosquito saliva (nothing odd there - so is almost everybody on the planet) and the active ingredient in poison ivy/sumac/oak (also extremely common)) but having something like an allergy seemingly come out of nowhere is nothing all that unusual - a person can go for years upon years with no troubles at all, then literally out of the blue, an allergy to <pick something> can develop with no visible cause or explanation. Case in point: Beekeepers - Getting stung while working a hive is pretty much "just part of the job", and typically doesn't amount to anything more than "Ouch... Next...".
Yet I've known two different beekeepers who have come close to dying after spending literally years working with bees and being stung from time to time as an occupational hazard. In both cases, there was no warning, nothing unusual, nada - Just one day they went out to tend their hives as they always did, caught a sting, and instead of it being the usual "ouchy itch" that could be ignored, each of them wound up in the emergency room swelled up like an overstuffed sausage and trying hard to drown in their own bodily fluids. Why? You tell me - if you can - so that I can tell them.
When it comes right down to where the rubber meets the road, an allergy is all about your immune system going over-the-top gonzo about something that most people don't notice at all, and potentially killing you with the over-reaction.
Why does it happen? Who can say - I have yet to hear a reasonable explanation from any source, be it crackpot, qualified, or anywhere in between.
What brings it on? Your guess is as good as anybody's.
What can be done about it? Very little other than treating the symptoms when a reaction happens, sometimes absolutely nothing.
How bad can it get? How bad do you consider it to be when simply walking into a room where a sample of the material that triggers an attack for you is sitting could literally be the cause of you needing to be fitted for a coffin?
As for your gal-pal, if I were her, my next move would be allergy-testing against her own dog's bodily materials - *VERY* lightly scrape the skin of a forearm - perhaps use a scrubber-pad like for dishes - you don't want to draw blood, just barely break the skin - and wipe a bit of his/her saliva on it, wait for a while, and see if there's any reaction. If not, try fluid swabbed from his sheath or her vulva, and so on. If no reaction from any of them, suspect something the dog she visited had on it - shampoo/soap residue, flea treatments, etc, or perhaps the dog's own secretions. Be *VERY* careful - suggest having an epi-pen handy during these trials, and 911 on the speed-dial - deadly-level allergies can kill in a matter of minutes. I'm not saying hers is that level, but do be aware that a mild reaction one time can turn into a full-blown case of anaphylactic shock the next time, and there's really no way to be certain unless/until it happens.
