Answer: Unfortunately, petiolaris and canadensis would both be used as descriptive or species names and without a genus it is difficult to research your friend's suggestion. (In a quick search I did not find anything with that combination.)
Here is a link to a photo gallery of fairly common weeds to see if any of them look at all similar. Perhaps you will see it there, although none pictured seem purple sometimes time of year and soil conditions can cause a purple cast.
Your local county extension and/or professionally trained nursery staff should be familiar with common problem plants that appear in your local area. I would suggest you contact them for help in identifying the plant. They may know immediately what it is, or they may appreciate a photo of the overall plant as well as closeups of the foliage. I'm sorry I can't be more specific for you.
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