If it was a v v v close friend I might say "I know it's not your DCs bday till whenever - is it OK if I drop his present in on the day?"
If it's a bday party I would send my kid with £10 in a card (aged 12). Or £5 in a card. But not nothing. Even if they turned up with a Mars Bar I wouldn't think anything.
I recently had a bday party (which cost about £45 per kid) for my DS and 2 kids from the same family turned up with nothing and said "Mum will drop his present round tmw" I said "how lovely to see you! Come in!" One of the kids from the same family same to DCs party last year empty handed.
TBH I would rather they turned up as they are good mates. I know their financial circumstances aren't great, but nor are mine. I also know that they eat out a lot and do an expensive hobby (which I won't name) at least twice a month.
I just wouldn't have the brass neck to send my kid with nothing.