I think a more appropriate translation would be 'friend for one', as it can be read as an offer. »Ven til en« is obviously an unusual way of phrasing it, but replace »ven« with any commodity you can purchase, then it sounds more like an offer (a free one, that is).

That being said, 'friend of one' is definitely an accurate translation as well. And 'friend to one' is just the literal translation. I wonder how they pronounce it, »ven til en« or »ventilen«.