Ring failure???? Wow, what a diagnosis. Is there a compression check to go along with that?

Does it smoke white or black? When does it smoke, all of the time, just when cold, accelerating, decelerating?

If there is a bunch of water in the muffler, stop the car on the steepest hill you can find and then mash the gas pedal to the floor and see if a quart of water pours out.
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Keeping it running is better than buying new

"1) Is there a test for the rings failing?"

A leakdown test


"2) What else might it be? 100k is nothing on a Volvo ..at least on my three RWD bricks!"

Don't know. What color is the smoke? Have you seen it? Maybe the mixture is off??

Ask your friend to take it to another mechanic or to a Volvo dealer. Someone who really knows Volvos. Unless the car has been seriously neglected (like no oil changes) there is very little chance that it is the rings. That is unheard with the 5 cylinder engines. The bottom end of the 5 cylinder engines is just as strong as a red block bottom end. If those engines have a weak point it is valve seats. But that is a problem that normally doesn't show up until after the engine reaches 200k+....