First off, "leave her better than you found her". So of course morals play a part in PU. Now some people's morals are different than others' morals, so that's the gray area, but it's not like you enter a "PUA" mode where you stop being a thinking, moral, human being, so...
Yeah, I definitely wouldn't mess with any situation in which the girl is living with a guy and they have kids, or are married, etc, etc. Wouldn't even bother flirting with them. You don't want to invite all that drama, and I think it's just wrong.
But short term and whatever you deem is long term boyfriends (1 year is what you would say?), is a different matter. To be honest, I have two different rules on it, one in either direction, and I oscillate between the two depending on the situation:
1. This guy is playing the same game as you, found the same girl attractive, and if he's doing a fine job, why break them up? To you, it's the same whether you get with this girl or with a different girl ten feet away who's equally attractive, but to him, it is a big deal, because he's already invested in this relationship with this other girl. I'm never one to break up something.
2. At the same time, you don't know their situation. It could be that he's treating her poorly. Some girls just don't want to be without a boyfriend, so jump from guy to guy instead of straight dropping a jerk. She's looking for an escape in this case, so why not make it be you? Either way, she's a grown adult, she knows the situation better than you, and she can make her own decision in this matter. Some people think that PU is just some form of manipulating a girl into doing what you want. That's not what it is. It's proving yourself an attractive guy who she is interested in, and letting her make the final decision. Even when using LMR techniques, if it doesn't work, are you going to rape her? NO! Let her "no" be "not yet", and just continue to prove you're worthwhile. It's simply like knowing how to navigate a woman's brain.
Hope that helps explain it!