Is one who gives a summary or explains an issue to others sinful if the others cheat on an exam using that summary, with the strong presumption that they will cheat or assist in cheating?

We do not know how a colleague's explanation of a matter can be a cause for cheating. As for providing summaries, even though their use in cheating is a possibility, this possibility cannot be definitively confirmed. Summaries are not the only means of cheating. The default assumption is the Muslim's integrity, and it is sufficient to advise anyone suspected of being keen on cheating to abandon it and strive hard in their studies.