invitation etiquette question

can't find this one in a book

I'm inviting one of DH's friends and his wife to my DH's surpise 30th Bday party. But, since I don't know where they live, I am sending the invite to the guy's parent's house, as:

Mr. and Mrs. John Smith
c/o Mr. and Mrs. Robert Smith

But, I wasn't planning on inviting the parents to the party. Is this a big etiquette faux pas?

(history: we invited this guy to our wedding with a guest because he was single at the time, and he never even RSVP'd. We did not invite the parents, but the guy still lived at home, so that's where we sent the invite. The guy eloped and his parents invited DH and I to the 'celebration', but not my MIL. We declined the invite)

I don tknow about that one, I cant find it in the book either!!!

Though, my FILs open mail that is in FHs name and c/o of them when they receive it, so they might open it. What if you just leave the c/o off? And right urgent on the front-or something to that affect? Maybe even send it and call him a few days later, if you know his number, and see if he recieved it

I dont know if I am even making sense...HTH