When I patch test clients i make them sign a form that states that despite a patch test a person can have a reaction even if the patch test is fine.
Explain that clients can happily wear lashes for years without a problem and all of a sudden develop a reaction.
When giving then after care I tell them again that despite the patch test on the rare occasion clients can have a reaction.
If they do I tell then what the symptoms are, that the first thing they should do is take an antihistamine, let me know, if no better after a day go see the doctor and worse case take them off.
Ask her to come see u or take a picture.
Tell her how to deal with it, be sympathetic to her.
If after that she is still being a pain then make it quite clear that you have insurance and that you patch test for those reasons so that in the event that somebody decides they want to sue that you have confirmed to your insurance and if she would like to speak to your insurance then she is more than welcome
She won't have a leg to stand on and was probably hoping for a pay out from you.
Horrible to say but I would never trust a client like this, people in this world are mostly ruthless.
Stand your ground, you have done nothing wrong, clients will react sometimes it's just life!