Absolutely, rats are a threat to chickens. Rats need to be trapped and killed. Along with trapping rats, good housekeeping will discourage them. Rats depend on a steady food source from spilled grains, accumulated feathers for their nests, and if they can access the coop and run, shelter. Rats have been known to chew body parts off chickens as the chickens sleep on their perches at night.
Your friend needs to plug all accesses with steel wool. Then clean all the spilled food out of coop and run. Then buy some rat traps. Care needs to be taken not to place these where the chickens can get to them, however, as rat traps can break a chicken's neck as they go to eat the bait.
How is your friend fixed for funds? I like those electric rat traps. They're easy to use, no messy mangled dead bodies to deal with, and they're much safer around chickens. But they're expensive.
Just don't use poison. It's too easy for it to find its way back to the chickens and kill them.