Hmmm, I can tell you a bit about my situation and if it is at all relevant/helpful, feel free to share it with your friend.
I married youngish (23)and I was full of romantic notions, a sort of naivety, a lack of experience of life and without a real sense of who I was and who I wanted to be. Therefore, the first few months were very stressful and did have an impact in a lot of ways. I think also, without wanting to sound like an amateur psychologist, that I really struggled with what being a "wife" was supposed to mean - my parents did not have a very strong relationship - certainly not one that I wanted to replicate - and they were much older than me when they married so I found it quite confusing to know what sort of template my marriage should follow. In all honesty it was friendship that got us through because my libido suffered, my identity as an individual suffered. We just had to talk (and sometimes yell!) it all through - what we wanted marriage to mean/look like. We ultimately decided that being married did not have to mean settling into a boring mundane existance and we had to make real effort to shape our relationship into something that we were happy with - we decided that we each had to feel free to do the things that we always wanted with the support of each other and with a few necessary compromises. And I had to take responsibility for the direction of my life - marriage or no marriage. We both worked hard at our very separate career routes for the first few years and then we each went back and did a Masters degree at University - we tried to do lots of budget travelling because it was something we both loved and we did our best to maintain our outside friendships.
We try to be open about sex - we have our ups and downs, but I am 32 and still married to dh, with a young son. We are a bit more settled (and boring) now (children do that to people) but I am so glad that, even with the compromises that were often necessary, we managed to work through the early marriage problems. DH is my lover, he is the brilliant father of our son, but the most important thing of all is that he is my closest friend and I think that by all standards we have a happy marriage.
Four months is a very short time to work at a relationship (dh and I were together for 5 years before we got married, so we had already dealt with alot of relationship issues before marriage) and marrying young means that you need to start compromising when alot of your peer group seem to have endless freedom (grass is greener syndrome). If your friends are positive that there is not even a remnant of a spark left then maybe it is better to let it end before there is a lot more to lose.
If there is still love and friendship and even a little bit of spark, perhaps they need to look at their individual expectations about being young versus being married and see if there is some conflict there...
I hope they can work it out...