@imsak,
Technically Im, we say "…one of whose friends….," though you might replace "have" by "had"
While "…very well known…" is awkward, as is using "friend" twice, and "…of mine…" is superfluous; maybe even "happened to". We'd be more likely to say something like
"Yesterday I bumped into a friend who I subsequently learned was the acquaintance of an even earlier buddy"
…depending of course on the exact circumstances