It is possible he's made 42% POT, but you need to know whether it's win, place or exotic betting.
There must be 1,000 selections to know whether it's going to continue as mentioned by my learned collegue ;-)

Divide the profit by the maximum win dividend and this will come up with a figure.

Divide this figure by the total number of winners and express as a percentage.

If the profit comes from less than 20% of the winners, then it probably is an anomaly.

My feeling is that your friend is either betting trifectas, or had a fluke run with outsiders on a small number of bets (less than 200) which won't continue.

A certain magazine which sells systems like this is proof it doesn't work longterm.