In modern parlance, word of mouth is a form of marketing. Advertising is “an openly sponsored, non-personal message to promote or sell a product, service or idea” [1]. (Also, word of mouth has strong incumbency bias. The printing press was revolutionary for a reason.)
There isn’t a single correct answer, because English. But in modern parlance, particularly in policy and commercial contexts, the paid part is key. “Word of mouth” encapsulates the difference: if they’re paid, they’re a spokesperson. If your friend pitching a product doesn’t disclose they’re paid, most would consider that dishonest.
In modern parlance, word of mouth is a form of marketing. Advertising is “an openly sponsored, non-personal message to promote or sell a product, service or idea” [1]. (Also, word of mouth has strong incumbency bias. The printing press was revolutionary for a reason.)
[1] https://en.m.wikipedia.org/wiki/Advertising