The comment was about short term trading (eg "day trading") I think. The reference to long term was in the context of regular short term trading over a long per

The comment was about short term trading (eg "day trading") I think. The reference to long term was in the context of regular short term trading over a long period. Investing in a market spread (eg an index tracker) over the long term with infrequent trading during that time is a different strategy that trusts the market rather than the individual's "skill". This latter is presumably what your relative was (correctly) advising.