It was inevitable that this was going to be a race characterized by rumors, bunk, lies, and disinformation. The closeness of the race makes fact-checking all the more important. Here are some resources that can be used to figure out what's real and what's not.
Tip: many of these sources will be viewed as being partisan, or being run by a "deep state." Where possible dig down and find the most neutral source (such as the state election commission), or a source that's more likely to be accepted by your friend.
GOOGLE - Google should be your first step. Because of the huge amount of attention on this subject, any disinformation is being very quickly exposed as such, often in great detail by multiple sources. Just type in a few key words regarding the subject, include the word "election" and you will often get multiple high-quality results immediately. For example, try "sharpie election"
FACTCHECK.ORG - Their main page has general election fact checks, and there's a page of Viral Voting Misinformation
[Update Nov 6, 2020] - New page: Trump’s Wild, Baseless Claims of Illegal Voting
FIRST DRAFT NEWS - Up to the minute tracking of disinformation. Focussed heavily on the election now.
CISA.GOV - The Cyberstructure and Infrastructure Security Agency is a US Government agency, which may or may not make it more palatable. They have a detailed section on Election Security, which can supply very useful context. There's a sub-section on "rumors" that focuses on disinformation.
SNOPES.COM - While you might a "LOL Snopes" response, and they do lean left, they have a great fact-checking team who give understandable explanations with references. Here it's generally best to lead with the facts in the Snopes article and save the link for later. Quick lists on their Election Day Live Blog, and Election Aftermath Blog. Use their search, or browse the "Hot 50" list, which is pretty much all election right now.
POLITIFACT.COM - Again you'll get pushback because of the source. Focus on the verifiable facts. Cut-and-paste and use screenshots rather than just dropping links.
BBC.COM/NEWS - Foreign news sites might be seen as a little more neutral. The BBC has a few pages of fact-checks. (Nov 20)
Site-Specific Searches - Many outlets are hard to browse for specific lists. You can use Google to search a site, and curate your own list. This is useful if you want to use a particular source because you know it will be listened to. To do this add "site:" and the URL of the site, like factcheck site:https://www.usatoday.com
JOURNALIST'S TOOLBOX - A large selection of additional resources.
TIME LIMITED IMAGE SEARCHES - A useful way of finding current information is to do a Google image search limiting the result to the last 24 hours. During developing events this gives you an easily scannable set of relevant results. Here's how:
Before limiting time, "sharpiegate" gives mostly hurricane-related results. Clicking on "24-hours" gives a full page of useful links regarding the election rumor.