Centre-back Steve Cook gave the Cherries the lead on 31 minutes with a scissor-kick finish, and Benik Afobe (79) headed in a second to secure a first win in four league matches.
Fourth official Friend was rushed to hospital after collapsing and banging his head. Friend passed out and received immediate treatment as a hush descended across the stadium at the tail end of the first half.
Referee Mike Dean quickly blew for half-time, before Friend was taken off for further treatment at hospital.
On the field Bournemouth secured their first win in five matches in all competitions, Cook and Afobe both profiting from Matt Ritchie free-kicks.
Toothless Southampton had few answers as Bournemouth edged further away from the Premier League drop zone.
Charlie Austin should have put Southampton ahead right from the off, with a snap side-foot from James Ward-Prowse’s cutback, but missed the target.
Ritchie and Josh King handed Southampton plenty of warning signs by sneaking in behind the visitors’ wing-backs, but Koeman’s men failed to heed the threat.
Saints paid for their disorganisation when King outwitted Jose Fonte on the right flank. Norway forward King first picked Fonte’s pocket, then forced the Portugal defender into conceding a cheap free-kick for obstruction.
Ritchie whipped in a dangerous ball that Forster could only parry, and Cook acrobatically hooked home to hand the hosts the lead.
Koeman’s response was immediate and merciless, Yoshida sacrificed and the 3-5-2 formation along with him.
Saints hitman Austin limped out of action just ahead of the hour, replaced by Graziano Pelle, and Shane Long hobbled out shortly afterwards.
Bournemouth doubled their advantage from another Ritchie set-piece, this time former Wolves poacher Afobe heading home his fourth goal for the Cherries to seal a precious victory.
Boruc, Smith, Francis, Cook, Daniels, Ritchie, Surman, Arter (Gosling 66), Gradel (Pugh 85), Afobe, King (Grabban 70).
Forster, van Dijk, Fonte, Yoshida (Steven Davis 35), Cedric, Ward-Prowse, Romeu, Bertrand, Mane, Long (Tadic 70), Austin (Pelle 60).
Mike Dean (Wirral).