Is it legal or illegal to take prescription drugs that have been prescribed for another person?

YES! Narcotics (opiates) are a controlled substance pretty much everywhere in the world. Sharing, selling or giving your medication (or for that matter, stealing someone else's) are all very serious criminal offenses in the US and Canada that can lead to arrest (that's doesn't necessarily mean your spouse would actually be charged though, that's for a judge to decide)

If your spouse is having pain to the extent they feel the need to take narcotics, they should see a doctor immediately. If you're concerned about your spouse showing signs of addiction, again your doctor can give you helpful contacts and information.