Plumbing with a combination of plastic and metal?

My wife's parents have a leaky joint in their bathroom where the water heater connects to the shower. It's a constant drip and they've been using a bucket for weeks (VERY small town, no plumbing services available). I have fixed many a leaky faucet in the past, including the shower in my own apartment somewhat recently which led her parents to ask me to help them fix theirs. When I went over to take a look I found the setup is a little unusual and I'm not sure how to proceed. They have a plastic pipe leading from an inaccessible hole in the wall (concrete, I'm not getting in without some heavy equipment) to a metal elbow joint, a small piece of the same plastic pipe leading to a second elbow joint, and then finally a third small plastic pipe leading into the shower pipe. All the connections and elbows are metal but the pipe leading in is plastic. I should also state that this is the hot water pipe, the plastic pipe is bright red (if that matters, I assume it's just to tell hot from cold - cold is white). The red pipe is approximately 1" thick, I didn't have a tape measure handy to get its exact measurements. Upon inspection of the setup I found that the elbow joints are heavily rusted as well as have considerable mildew/mold (green and white solid-ish sludge) all over them. They weren't budging and I was about to back out of trying to fix it for fear of shattering one of the plastic connection pipes and causing a drip to become flooding (water was turned off of course but turning it back on would result in bad news). I was told to proceed and so I did, sure enough one of the pipes formed a crack and I stopped. They turned the water back on and it didn't leak (surprisingly, it didn't even drip) and I'm going back in a few days once I've had a chance to get to the plumbing shop. Okay, so that's the story. The question (or questions) I'm looking for is.. I can't replace the main piece of plastic pipe, though at the moment it is fine, no damage at all. All the rust/sludge buildup happened past the first pipe's connection, leaving the pipe fully intact. I have to replace both elbow joints and two small pieces of plastic pipe, should I replace them with the same types as they have now? Is there a better option? I'm not familiar with connecting metal to plastic. I have a pile of washers but I seem to recall my mother telling me years ago something to do with plastic and plumber's tape. Do I need something besides a simple washer for this project? Worst case scenario is they have no water for 24 hours and call in an out of town plumber. But I'm hoping I can resolve this for them without it coming to that.