My 80-year-old grandmother has Russian citizenship (she got it because she was 35 ...?

At the moment, there is no direct ban on Russian citizens entering Ukraine.

According to the agreements, there is a visa-free regime between Ukraine and the Russian Federation. However, to cross the Ukrainian border, citizens of the Russian Federation must have a foreign passport, since entry to the territory of Ukraine with internal (Russian) passports was canceled back in 2015. According to the decision of the National Security Service of Ukraine, enhanced biometric control has been introduced at the border, especially for citizens of countries included in the migration risk list, which also includes Russia.

According to Ukrainian legislation, the maximum period of visa-free stay in the country for citizens of the Russian Federation is 90 days within 180 days from the day of first entry. In other words, Russians can stay in Ukraine for three months out of six, after which they must leave the country. The right to re-entry will appear only after 90 days.