The gold standard, at least in the US, is Art of Problem Solving. https://artofproblemsolving.com/ https://artofproblemsolving.com/ For the most part it is no

The gold standard, at least in the US, is Art of Problem Solving. https://artofproblemsolving.com/ https://artofproblemsolving.com/ For the most part it is not free, but there are some free resources https://artofproblemsolving.com/resources https://artofproblemsolving.com/resources 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. 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. Here are some free notes on a variety of topics from the American Math Society: https://www.ams.org/open-math-notes?grad_level=5 https://www.ams.org/open-math-notes?grad_level=5 Here is my own contribution -- 150 pages on combinatorics, probability, and modeling based on TV game shows: https://www.ams.org/open-math-notes/omn-view-listing?listingId=110856 https://www.ams.org/open-math-notes/omn-view-listing?listing... Best of luck!