As someone who a) Lived through the period, and b) Studied East European history to Masters level, this statement is absolute bollocks. There are a number of long and short term causes on why the Communist regimes of Eastern Europe failed but in the main the decision taken by Gorbachev not to interfere with the internal politics of those countries was the deciding factor. As a consequence either the local political elites moved towards democratic elections, as in Poland or Hungary, or the people either tried to leave en masse, as happened in East Germany or rose up in arms, as in Romania. It is incredibly disingenuous to say Thatcher was in anyway involved other than being opposed to those regimes.
I think, to some extent, that Thatcher's friendship with Gorbachev and, following from that, Gorbachev's friendship with Reagan, formed a triumvirate of countries which trusted each other and wanted to bring about the end of the Cold War. I know Poland well, and it was always a country of Western ideas, but shackled by Russian Communism, which it hated, but had to abide by. Lech Walensa started the ball rolling with Solidarnosc but the Reagan-Gorbachev-Thatcher triumvirate gave East Berlin the confidence to chip away at the Berlin Wall until it finally fell. Mrs Thatcher didn't single-handedly end the Cold War, but as Seagull on the Wing said, she played some part in it.