Some people are born with allergies - some develop them later in life. People who are allergic to peanuts are more prone to also be allergic to tree nuts than the inverse. But, there is no set rule. While it is ironic that you would know two people who developed a nut allergy in a short period of time - it's not a staggering statistical improbability.
As for the sources and type of almonds - there are two types of almonds, sweet almonds and bitter almonds. The almonds that we eat in the US are sweet almonds almost certainly from California. Bitter almonds found in other parts of the world are illegal to import into the US (if I remember the import restrictions correctly) because they contain a very potent poison amygdalin, which when mixed with the enzymes in saliva create hydrocyanic acid, which causes death by cyanide poisoning - a lethal does is only 20 almonds for an adult, less in children. Bitter almonds are used in the US for making almond extracts because the amygdalin is removed during processing.
As for it being a reaction to chemicals used in the field such as pestacide sprays ... this is very unlikely - as it's a seed inside a nut inside a fruit. The only chemical I can think of that they might be treated with during processing would be the oil used in blanching them - if they were blanched nuts. Ironically, this could be coconut or peanut oil, or a blend, which can also cause allergic reactions for people who are sensative/allergic to tree nuts.
I would stronly suggest your friends stop eating all nuts and talk to their doctor - who can give them a referral to an allergist who can test them. I couldn't begin to count the number of times I've seen patients show up in the ER with severe allergic reactions to something they had eaten all of their lives without a problem.
Allergies are funny - you can outgrow them - or you can develop them.