I shook the hand of a man who previously suffered from necrotizing fasciities (resolved, had scar) and suffers from lupus. can i contract nf this way?

Probably not: Without an open sore, ingesting the bactteria or a compromised immune system you probably would not get it even if his infection were active which it does not seem to be form your description.

Not really.: If you have some form of immune problem, you might be able to acquire bacteria easily this way, but, for a normal person, the bacteria that cause nec fasc will need to get under the skin first, to cause trouble. If you don't have a cut or break in the skin, it's very unlikely to be a problem.