Eh, that's a bummer. Does she have any issues with yeast? That could cause this. Here is an older thread where a doctor on medhelp weighs in https://www.medhelp.org/posts/Dermatology/Penis-irritation-after-sex/show/241569. He
states that this would probably not be an allergy or anything like that because it would cause scaling and itching and wouldn't go away on its own after. He also says not an std type of situation (which you probably know).
His suggestion is that it is just from the friction where a condom would normally cut down on it.
Putting shea butter and vitamin E on it might help.
In that thread I linked, there is something that used a product too that helped them.
I'm assuming this is without condom?