Did you use your phone (or any device) at their house? Use their Wifi?
It's likely they've linked you to them that way and therefore you're seeing some of their targeted ads, which given they've bought this cup before is likely something they've looked up.
You know, just as folk rationalise to pander to their delusions, the same can be said for the opposite. Unless you are deliberately playing Devil's Advocate (which is sound), how can you see this as anything other than a typical case of hotwording?
Don't you think that it is an even larger coincidence that the bloke was actually talking about it? I mean, the word coincidence has been thrown around here quite a lot - but 9 pages on, are you seriously still thinking that people's experiences and stories are just coincidence?
The simplest explanation to this is simply that his phone picked up on buzzwords (linking brand names, etc...) rather than well... If I gleaned this right - he signed in on their Wifi, so their friends' searches and purchases have somehow infected his phone... and out of all of the hundreds and thousands of items they could have searched for, (or others), 'it' has chosen to throw this one advert at him?
Or.
A device that has a microphone on it (that reads speech very well), that is carried around everywhere, has a number of top 10 app downloads on it and the resources to monitor hotwords at the end-user level, has picked up on a 'vocal cookie' and linked an advertising campaign to him.
I am astounded that folk still think that... well, I will quote the mother of all cynics:
No they aren't.