Can I bring maple syrup into Spain??

No one here can give you a definitive answer. Maple syrup is just sugar and water (like Coca Cola, only thicker) so as long at it is in checked luggage, you should be OK. I would not lose any sleep over it.

I don't know about customs (though I think it should be OK in checked luggage). But do you know if your Spanish friends even like maple syrup? I have Spanish friends who have visited here and tried it - and absolutely hated it. (My friends also don't like the taste of butter, since they are more accustomed to things cooked with olive oil.) Needless to say, the visit to the oh-so-American pancake place was a big flop.

We had an exchange student and she absolutely fell in love with maple syrup so we'd like to bring some with us when we visit her family.

Countries try to restrict plant materials that may spread disease to native plants. You do not have a raw plant material as the sap has been boiled down and processed. You have nothing to declare and should just walk through customs.

I'd suggest an unopened, sealed original bottle in your checked luggage. Hopefully (unless you're from Vermont) it's not something you've done up yourself. FYI I took "five alarm" chili mix as a typically American gift to Dutch friends once and they hated it. The next time it was maple syrup and they loved it.

If your friends in Spain are anywhere near an Iceland supermarket they carry wonderful Tate Lyle maple syrup .