My only assumption was Instagram/Facebook was somehow listening our conversation.
If the answer to either of those two is "yes", and if your friend googled or liked/followed horse racing content on instagram/fb, then the algo can easily determine that indeed you might be interested in it and recommend it to you. Requires zero horse racing related searches on your own devices, just requires a person with a first degree connection to you doing those searches.
If the system knows that your close social link with some similar interests interacts (searching, reading, liking, following, chatting about online with others, etc.) a lot with a specific type of content, it is a pretty straightforward idea to suggest that specific type of content to you, even if you didn't interact with that type of content online yourself yet.
Since we don't leave any apps open on our Android phones, we assume it's because Google was listening (their version of Siri is Bixby, which we use from time to time) then selling that information. It was probably a one-off, since this was only for a short window of time (2 weeks), but it was blatant.
So you force stop everything?