Tell your mom not to worry! The USA visit noted in her passport will pose no problems when entering Cuba. As a general rule, Cuban Immigration will not stamp your passport, so if you want it stamped, make sure you ask.
Cuba in fact welcomes visitors from the USA and many Americans travel to Cuba via Canada or Mexico. It's the American government that has issues with its citizens traveling to Cuba and depending on the situation, Uncle Sam levies some pretty steep fines if they get caught.