Abandoning citizenship usually requires jumping through hoops, and I wouldn’t be surprised if she had to return to Russia to file the paperwork.

In other instances, though, it’s actually really easy to inherit multiple citizenship, especially if one of those is American. You’re automatically an American citizen if you were born in the US or if either of your parents was an American citizen at the time of your birth. Additionally, anyone born to two Russian parents is automatically a Russian citizen, or if they were born in Russia to at least one Russian parent. So if a Russian couple who went to America after the USSR collapsed but didn’t bother renouncing their citizenship and then had kids, those kids would have both Russian and American citizenship. Alternatively, if an American citizen went to the Russian Federation and had a child with a local, the child would also have dual citizenship.