Jus sanguinis is what Germany used to use to determine citizenship, but I think very recently that has been changed somewhat. As I understand it, you cannot keep this right if you get other citizenship like you did in 1978.
My guess is no they cannot get it, but I could be wrong. I don't know of any benefits other than travel type ones ie EU Visa. My brother in law has German citizenship, and is keeping it in favor of Canadian for this reason. I don't see it as worth it personally.