On dead legs part 135 ops can go under part 91 and only then could she can log dual received if the PC had the approproate CFI certs. How often they get no revenue legs is another story. As long as there is revenue onboard she cannot log anything.
There are a few low-life operators who prey on pilots looking to build up time and scam them into buying F/O time as SIC flying under part 135 ops. (Basically Pay For Training or buying your job.) This can be very sticky as quite often the a/c does not require a SIC, therefore unless one is required per the airlines operations specs, you cannot log SIC time.
Check out
http://forums.flightinfo.com as there are topics about those subjects.