Cum allergies would not mean that creampies are OK. Usually that would also end up with rashes and symptoms wherever the load ends up.
So if allergic any cumshot should be avoided.
Though you are not allergic to all types of cum. You can be allergic to one person but not another. And you can also have allergic symptopms taking a load from a person who ate something you are allergic to.
And technically you are not allergic to the cum itself its the fluid around it that contains the protein that your body reacts to.
I dont doubt her being allergic, its actually not that uncommon. Its just not a valid reason for getting a creampie instead of swallowing.
Theres a reason why there is non-latex condoms.