Barely a decade after the American and French victory against Britain, Poland was partitioned for the third and final time, being mostly subsumed into the Russian Empire.
During the American war, two Poles - Kasimir Pulaski and Tadeuz Kosciuszko - played a prominent role in securing a victory. Pulaski died for the American cause, and Kosciuszko was deeply interested in American politics and republicanism.
How did the Americans react to Poland's demise? From an ideological standpoint, the conquest of a semi-republican state by an autocratic empire presumably would have horrified many statesmen. And from a diplomatic standpoint, the destruction of a country that produced people like Kosciuszko must have seemed tragic.
Do we know what President Washington thought about the event? What about other American statesmen? Was there any kind of official national response?
I can't answer your question in full, but I can at least address part of it. Washington was aware of the third partition of Poland and corresponded with Julian Ursyn Niemcewicz, a friend of Tadeuz Kosciuszko, in 1798.
Niemcewicz, a Polish writer and statesman, had traveled to the United States and visited Washington at his home in Mount Vernon. He later wrote to thank Washington and his wife for their hospitality.
The political situation in Poland was mentioned only briefly, but this is the key part of Washington's response:
That your country is not as happy as your struggle to make it so was Patriotic and Noble, is a matter which all lovers of rational Liberty and the Rights of Man, have sorely lamented: and if my Vows, during the arduous contest could have availed, you would now, have been as happy in the enjoyment of these desirable blessings under your own Vine and Fig tree, as the People of these United States may be under theirs.
You can find the full text of the correspondence here.