@Forever_Jung - Yes, I can relate!
In terms of growing through it, I think with time (Si?) I have come to realize more and more that when I look back on the past, the most meaningful and satisfying elements of my life are those that I have stuck with long enough for them to become an integral part of me. I try to look at it in less of a short-term view and more of a long-term: maybe I am not SUPER HYPER EXCITED about our relationship every single second, but when I look back on the year, it makes my heart glow. And in terms of the moments that are not so great, they are good opportunities for growth, for exercise of patience and compassion. Part of being in a long-term relationship I think is learning more "agape" love and less "eros" love, where agape love is more altruistic. I will connect to that more in a second.
I am in a relatively happy and stable relationship, over 2 years at this point. Sort of funny, he is a steady, easygoing 9. I have had my fair share of questioning the relationship and as a 6 do hyperanalyze it fairly frequently. I actively questioned it a lot especially towards the end of our first year, when the honeymoon phase was ending and we were becoming less like excited, infatuated lovers and more like stable, constant partners. I think that is a natural transition, and there is still abundant spark between us, but it does make a big difference when, as you said (and nice metaphor!) the relationship is prose instead of poetry.
I do not think I will ever be free of the next-best-thing disease, but I now I try more to orient myself towards looking for new experiences in concert with my commitments, if that makes sense. So, if I am feeling frustrated in the relationship, instead of emotionally distancing myself from my partner and seeking stimulation elsewhere, I try to figure out something for my partner and I do to bring us together and kindle some fire. We are naturally very different people, which is part of why we find each other so compelling, but it also means that we are not always hardwired in a way that will automatically do or be exactly what would be most pleasing to each other, and sometimes that connection has to be actively worked on. I think the NF mindset tends to be that if something is meant to be, it ought to fall together perfectly, and we tend to neglect the gritty legwork because it doesn't sound like part of the fairytale. But ENFPs love projects and get a huge emotional high out of anticipation, so plotting a big adventure with your partner is such an awesome solution. It redirects all those Ne pathways towards "ways I can be happy in this relationship" instead of "ways I could be happier outside this relationship".
The other helpful perspective to me is what I was mentioning before about altruism. It hit me one day that basing my emotional commitment to my partner on my moment-to-moment feelings of happiness is super... well... selfish. I do believe that I deserve to be happy and that I should not settle, but it is also very one-sided to value how meaningful my partner is to me based on whether the relationship is making me ecstatically happy all the time or not. I am trying harder to stop making it all about my happiness, because that just makes me miserable and it certainly doesn't help my boyfriend.
So together, my two "weapons" against the threat of grass-is-greener syndrome are, one, creating a happy future together and not just being upset when it doesn't fall in my lap, and, two, stepping away from my obsessive focus on satisfaction and doing kind things for my partner without regard to my personal gain because I love him and want him to be happy.
Good luck to your friend... and good luck to you... this is really sweet of you.