"While he may be lying by omission, I think he is doing this to give himself an easy life and build up their trust as a couple"
Really?
Trust built on lies to give himself an easy life?
She doesn't accept him exactly as he is, because he is lying to her so she doesn't know who he is really.
Does it suprise you he's taken a long time to find someone?
The thing is operationsunshine it's not up to you to manage his relationship for him by backing away.
None of this is on you, it's on him.
Fair enough, he won't be told what to do and who to see, he's independent - great.
But if he doesn't naturally want to spend time with her rather than you, if he naturally lies to her by omission for an easy life, if they've fought over your friendship and his solution is to lie about it, he's a really crappy boyfriend/ partner.
It's not on you, it's on him - but his girlfriend is putting up with an unsatisfactory, substandard relationship in which she's lied to and they're not long together.
She'd be well advised to break up with him and find someone who treats her better, but she doesn't have all the facts because he's lying to her.