Hi @Digon !
Your daughter shouldn't worry. I don't believe this is connected, and I didn't experience any infertility related to histamine intolerance following my embryo transfer. I worried about this a lot myself, but it turned out there was no need. It's really sad that one has to research this condition in forums. There aren't many studies on this, so allergy drs/specialists are very cautious when it comes to advising. If it gives her any peace of mind, she should have a low histamine diet (unfortunately this diet is pretty bland: breakfast you can do a smoothie with oatmilk, mango, spinach and oats. Lunch/dinner: poultry, salmon and a low histamine sallad/rice or a jacket potatoe with cream cheese is what I would recommend). You are quite restricted in restaurants unfortunately. You can google low histamine diet, the link above is pretty accurate in classifying foods in low, medium, high histamine. Rule of thumb is: classic allergenic foods like gluten, shellfish, exotic fruits, nuts, soy, tomatoes, canned food, alcohol, fermented foods etc. are high histamine. Many dieticians support this diet.
If she is successful, she should continue eating a low histamine diet for a few weeks, I would play it safe until week 12 . My allergy dr specialist told me not to overdo it on the DAO enzyme tablets, but you can certainly take an antihistamine tablet. Google which ones are safe on NHS website. BUT around week 10-16, the body starts producing up to 600% more DAO enzyme than normal (something she would be deficient in if she has histamine intolerance). The body does this in order to build a protective DAO enzyme layer around the baby. Her symptoms will probably subside. Turns out getting pregnant was for me a way to permanently get rid of histamine intolerance!
I am free of my histamine intolerance now, and have reintroduced many foods I had to avoid before and in the beginning of my pregnancy. I was successful on my second attempt, and have a healthy, beautiful 5 month-old daughter. Good luck, wish her all the best! Let me know if any further questions, however I do want to stress I'm not a dietician or medical professional and can just share advice based on my own experience.