Now I'm only really a social partner dancer. I haven't entered a competition for ages, and for me dancing means leading a girl around the floor. So if you want to know about street dance, belly dancing or ballet then I'm not really qualified to answer. So here is my answer for him, slightly edited. I hope others find it useful.
So in summary in terms of difficulty: modern jive easier than salsa, which is about the same as west coast swing, which are both easier than Lindy Hop, which is probably about the same as ballroom. NB, this is difficulty to start to learn, not difficulty to master,