It isn’t just a DWI. A lot of other infractions - including some serious and some that the US has often considered sort of ‘youthful indiscretions’ (such as reckless driving, speeding above the max limit in that state, racing another vehicle, etc.) - are a bar to entry into Canada.
Your ‘friend’ may have screwed him/herself for AT LEAST five years from the date of the completion of their sentence, even if the initial charge was subsequently reduced to a lesser offense.
As of December 2018, a DUI is a serious crime in Canada and such an offense no longer qualifies for automatic Deemed Rehabilitation after ten years. This significant change is due to the Government of Canada implementing new DUI laws that increased the maximum length of imprisonment to a decade. Consequently, impaired driving offenses are now considered too serious to qualify for Deemed Rehabilitation by virtue of time, and an American with a single DWI can now be denied entry at the Canadian border even if the offense occurred more than ten years ago.
Oftentimes it is easier (although not necessarily easy) to get the US conviction modified or recategorized, although even that may not allow them to subsequently get Canadian entry depending on the wording of the individual law and potential max term of punishment in the individual USstate where the infraction was committed.
Your ‘friend’ needs to talk to an attorney and that’s going to be costly.