No, but he would be. She would merely be in possession of it.
For the record, Denmark has, indeed, punished a large number of underage offenders for spreading child pornography through social media. It's mostly been regarding clips spread without the participants' consent though.
Not sure whether other countries go by the age of sexual consent, but I've personally never heard any other definition of child porn than "sexually explicit/suggestive content involving people under the age of 18". Assuming that's valid everywhere, then it would indeed be child pornography since the Dane was under 18.
EU-wide harmonization of such laws would be a welcome step to take before privacy-invading measures were to be taken, and such legislation definitely should address underage sexting.