Are they a foreign national that is a NASA employee and sent as part of their job? If so, then absolutely NASA is paying.
As for the employees of other space agencies, speculation on my behalf here, a quick googling of NASA documents didn't reveal anything. But I would not think that NASA is paying for flights, why would they?.
As examples, For 2021, the European Space Agency has a budget of $2.283 billion (USD) and the Japanese Space Agency has a similar budget, they would be budgeting for the cost of astronaut stays years in advance, including their share of the taxi ride, resupply missions, years of training etc. They'd also be paying for a share of maintenance costs for the ISS, tracking and communications and costs of whatever experiments they choose to send up there and conduct.