A quick Google search (actually I use Duckduckgo but "a quick Duck or DDG search" would sound weird...) showed that "friend's" was a bit more common, whereas a Google user manual or tutorial used "friends' " and I (maybe biasly) assume them to use (more) correct grammar. The sentence used in the Google user manual was:
"See friends' locations on a map"

(see this link https://support.google.com/plus/answer/3302509?hl=en)

  • Sorry bout that. – Rasmus Mar 27 '16 at 12:49
  • No worries, dupes are sometimes hard to find. If the suggested dupe answers your question, please click on the "That solved my problem" link. – terdon Mar 27 '16 at 13:01

Friend's means of a single friend, friends' means of multiple friends. General rule: for a single form - noun+'s, for plural form - noun+s'.

  • Thanks, it seems so obvious now...And, well, it actually is. – Rasmus Mar 27 '16 at 12:48

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