An aquaintance of mine tells me he has made a profit of 42 percent on a system he has devised.Naturally he won't release what it is(as though my bets would make a difference)but I don't know whether he's pulling my leg or not.Do you think this profit is possible.I would appreciate your comments,thanks in advance.

Certainly sounds plausible to me.
I came up with a system that had over a hundred percent profit.
I devised it using past results.
But when I applied it to future events,I lost my money just the same!
Ask your friend how many race results are included in his sample.
I would think a minimum of 1000 results means he is fairly safe to continue applying his system.

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.