dragonflysky, I think it's a sad and unfortunate situation, but it seems to me it's an example of how time isn't just used on other partners, since apparently he stopped having time to see you due to his job and parental obligations and not due to "using up" all his love on the one partner.
My boyfriend has only me currently, but he also has a very demanding job. He gets up at 7am and has been at work as late at 4am at times, although the usual is closer to 11pm. When he gets back home, he doesn't have much time to do anything other than sleep, and if we didn't live together at the moment, I wouldn't see him at all during the week.
Then during the weekend, everything that he has to do catches up with him since he couldn't do any of it during the week, and I tag along but here again, we probably wouldn't be able to make plans for dating if we didn't live together.
I'm sure some people have a much less demanding job that takes less time out of their schedule, and they can give their partners more time and attention, even if they have more than one, even if they don't see them at the same time.
I'm satisfied with the situation as it is because I'm someone who also likes having a lot of alone time, and I always fall for the busy guys, so I know if he stopped being busy it might work less well between us. But I can see how that wouldn't work at all for some people, and it doesn't have much to do with polyamory. If your friend didn't have his current girlfriend, it's possible he would have told you the exact same thing.