They were alerted by a taxi driver who left the pair home.
When they arrived, the woman flagged police down, saying the man had chased her with a shovel.
PSNI Foyle took to social media to thank the taxi driver.
They said they 'dread to think' what may have happened if they weren't there.
He is due to appear in court in connection with the incident.
O’Neill and Little-Pengelly urged to act on alcohol price limits as deaths rise
£14,000 pay increase recommended for Northern Ireland Assembly members
Police receive more than 100 referrals in Presbyterian Church safeguarding case
Bus driver dies at scene of crash involving school children
Jury sworn in at Natalie McNally murder trial