Or, get together an army with sufficient siege equipment (culture fortifies a city, and will need to bombarded down if your troops are to stand a chance) and capture and/or raze their high-culture cities. The decision whether to capture or raze is very situational. By razing, you may be destroying some tasty wonders, but on the other hand this can allow your own culture-producing cities to push their borders out over the new "no man's land", and new settlements can't hope to compete with the cultural output of an established city (unless, of course, they/you deviously throw down a culture bomb - aka great work - in the new settlement).
i don't play MP except coop with hubby so i can't give real advice except to say good luck
i think that what your friend is doing is not what TravellingHat is talking about at the very end of what i quoted above. but, since i just saw a version of what he's talking about in action, i can tell you a little more detail about it. i ramble, so the long story follows, but the short version is: even in brand new cities you just captured, using a great artist to make a great work can (1) end the revolt instantly, (2) grant you border pops and therefore cultural control of the tiles, making your enemy not able to use roads that might be there already, which gives you time to heal and/or bring in more troops since he can't reach you as fast, and (3) grants you an instant and significan boost in cultural defense. great artists used strategically are incredible combat units basically.
earlier today i had Louis (the AI, not multiplayer, of course) down to two cities. he'd been saving up great people for a golden age, and had built a settler, and at some point built a third city, and took along a great artist i guess. i took one city. then i took what used to be his last city. at that point, i would have been able to take his newest city, the one built by that settler, in 2 turns. there were roads connecting it with the city i'd just captured, but the culture connecting the two cities was mostly gone, meaning i could use them. the city was 2 population, had to have just hit 2 recently, he's a creative leader and didn't have any or much cultural defense. that was why i was calculating 2 turns to reach it, i wouldn't need my one-movement siege weapons, i had enough 2-movement stronger units and could take it without bombarding/collatoral damage was my theory.
then he used the culture bomb/great work weapon after i hit enter. it was a brilliant move on his part! he got three rings of border pop, and 60% more cultural defense, so now i did need my siege weapons, and would take 4 turns to get there (no using roads, and having to bring along 1-movement weapons too). i did get there of course. he had his own siege weapons in there, hit me with them a little bit, some retreated, but then he died. the best part was ... when i captured that city (it was very well placed so even at population one i wanted to keep it), it had a military academy in it! those retreats must have given him the exp he needed to earn a great general
he did this in his own city. he didn't even need the "get the city out of revolt instantly" effect i use it partly for myself at times. but it still helped him quite a bit. it didn't save him, he had no hope...he was down to one tiny city, what could he do to really save his empire at that point? but he did the smartest thing he could, and made my job harder, and i cheered him for the effort.
