This question already has an answer here:
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)