The gold standard, at least in the US, is Art of Problem Solving.<p><a href="https://artofproblemsolving.com/" rel="nofollow">https://artofproblemsolving.com/</a><p>For the most part it is not free, but there are some free resources<p><a href="https://artofproblemsolving.com/resources" rel="nofollow">https://artofproblemsolving.com/resources</a><p>and the paid programs don't cost too much, something like $400 for a 12-week online class. Also, the times are generally chosen to be convenient in the US, which might be difficult for your friend depending on where they are.<p>If you genuinely want something free, there is a ton out there, much of it good, but you might need to give up on "from reputable institutions". One person might post something online as a labor of love, and often that person might be employed at a reputable university, but if you want something backed up by an organization as a whole -- especially something more than "here are some notes you can read, or videos you can watch" -- then that organization will usually be asking what they stand to gain.<p>Here are some free notes on a variety of topics from the American Math Society:<p><a href="https://www.ams.org/open-math-notes?grad_level=5" rel="nofollow">https://www.ams.org/open-math-notes?grad_level=5</a><p>Here is my own contribution -- 150 pages on combinatorics, probability, and modeling based on TV game shows:<p><a href="https://www.ams.org/open-math-notes/omn-view-listing?listingId=110856" rel="nofollow">https://www.ams.org/open-math-notes/omn-view-listing?listing...</a><p>Best of luck!