Yes, the West and other countries should help Belarus pay the costs of Chernobyl, because it would be a sign of good faith with the country. Belarus is very poor and nearly 20 percent of the GDP gets eaten up with Chernobyl expenses. The United States could help Belarus make ties with the West.
Belarus is a very small country with limited resources. Richer nations that have money to spend on foreign aid should certainly consider sending some to Belarus. The Chernobyl disaster is one of the most famous of its kind, and sometimes some good will is needed to strengthen our common bonds.
At the time of Chernobyl, other countries visited the site, UN forces helped evacuate people living in the danger area and help came from across the world. The time to ask for help was then. To ask for help 25 years later, as parts of the zone become liveable once again, seems cheeky, and shows a lack of planning on Belarus' part.
Chernobyl was an incident that happened on then Russian soil and other countries should not feel obligated to pay for reconstruction costs. If other countries wish to help with the costs then that is their choice but they should not be required to. Chernobyl should be a lesson to the world of the destructive power of nuclear power.