I have been giving this question some thought and I have come to the conclusion that the answer should be completely left alone without any comments or edits to add sources.
Although @ArnonWeinberg pointed out in the comments that
The post notice reads: "Posts with unsourced content may be edited or deleted." - worst case, the author can roll back edits, but usually people don't object to having their posts improved.
The reason for this is two-fold.
- Editing the answer enables the answerer to keep saying "I'm too busy" or "I can't be bothered with that" and not provide their own sources of fact.
- It can send out the message to others that it doesn't matter anyway because others will provide them.
This lowers overall standards in the community. Instead, I think we need to strengthen our community standards with this as there are a lot of unsourced answers on the site.
I personally think that if you have found sources, which back the facts given, provide them in your own answer. Maybe you can add to that answer at the same time.
Seeing that, I would upvote yours and deal with the other one in either of two ways.
- If the unsourced answer is from a new member of the community I would initially leave it with no up or down votes and comment asking for sources.
- If the answerer has already been nudged for sources and a couple of days has passed, I would downvote the answer and vote for deletion or flag for low quality. Sometimes the low quality flag is not available for some reason, so I either pick a closer flag or flag the mods for their intervention.
This has been my approach to answers throughout the site, whether other answers are provided or not.
It may be correct, but it may not. The bottom line is that it is not sourced, and sources not only back claims, but make the answer useful to those learning about the subject area covered. Those learning could build beliefs based on incorrect unsourced information and that is not a good idea. Especially in such delicate areas of study we cover.