My DC has a severe nut allergy. Nuts were (before we found out when he was a baby) my absolute favourite thing. I ate them all the time and was gutted when he turned out to have an allergy to them but the second i found out that being around them could kill him, giving them up was not an issue, although I missed them initially.
Weirdly now the thought of eating the particular nuts they are allergic too makes me feel sick, even the smell of them. If it was my DH with the same allergy, I wouldnt hesitate to not eat something that could make him physically ill. Having seen my DC have an anaphylactic reaction, I would never knowingly put anyone through that. However this has been a battle ive had to fight for my DC many many times over the years , even family members still bought sweets with nuts in, given them food with "may "contain" warnings and generally just dont get it. I had to make a formal complaint about a club DC had attended for many years who fed a group of children peanut butter sandwiches and though washing the knife before cutting their sandwich was adequate. They just didnt understand cross contamination and that that could lead to a full blown reaction. That's just one example, there have been many of the years.
ive found if people dont have to live serious allergies everyday, it is really hard to get their heads around and just washing the knife sounded reasonable to the person who was leading the group.
This did lead to a really good discussion about allergies that helped keep more then just my DC safe at other events and involved AAI (Epipen etc) training for the people in charge.
Allergens stay in your system for 24 hours, no amount of tooth brushing, handwashing etc can get rid of it. One of my biggest fears in life is that my now teenage DCs first kiss may kill them if the other person has been eating nuts in the day beforehand . Thankfully we have taught them to advocate for themselves well and they aren't worried about saying please dont eat nuts around me and explaining why its important not to.
I really feel for you OP, it sounds like prior to this your DP has been understanding, hopefully he wont let his DM influence him further, you have to advocate for your own needs if its genuinely a matter of life or death. I wish people understood severe allergies more.