Generally yes, unless exempt by treaty.  What are the codes shown on the 1042-S?

Here are some of the questions that need to be raised.  Which country was your client a tax resident of?  What visa was he on?  Was your client in the US supporting his/her countrymen during tournaments in the US?  How many days was he/she present in the US for any purpose during the calendar year or a rolling 12-month period?  Was the legal employer a US or foreign company?  Was the cost of the remuneration borne by a US or foreign company?  If a foreign company, does that company have a PE in the US (this would be a more involving question beyond just the individual)?