Your friend's dilemma, so common these days, makes me furious. It is outrageous that women are put in the position of being forced to agree to major surgery in order to obtain medical care, and those who perpetrate this outrageous violation of medical ethics and human rights can get away with it without so much as a ripple in the public consciousness.
But let me take a deep breath and calm down. More to your point, I don't know as your friend's age has anything to do with a healthy pregnancy, provided, as you say, she has no health problems already, or with the uterine scar. I do know that out-of-hospital birth gives your friend her best shot at a complication-free VBAC because she would be less likely to be subjected to policies (time limits, for example) that decrease her chances of vaginal birth and procedures (oxytocin to make contractions stronger, for example) that can increase her risk of uterine scar rupture. In the sole out-of-hospital study of VBAC, the odds of scar rupture were 4 per 1000 and the likelihood of VBAC in a woman with no prior vaginal births was a whopping 81% (Lieberman 2004)! In hospital studies, the odds of scar rupture are often double that rate or more, and the odds of vaginal birth even in populations where substantial proportions of the women have prior vaginal births are rarely this high. The fact that four years have passed since her cesarean is also in your friend's favor. Intervals between births longer than 18 months decrease risk of scar problems. Notice, though, that the odds of scar rupture are not zero. And while the scar giving way rarely results in harm beyond the need for repeat surgery, the odds of harm to the baby are not zero either. Also, one piece of missing information is whether single-layer or double-layer uterine suturing was done at the cesarean. Single-layer suturing may be associated with a somewhat higher risk of scar failure. Still, it comes down to this: only your friend can decide whether she feels the risk is acceptable. No one can guarantee her that everything will be all right. That, of course, is true under any circumstances.
-- Henci
Lieberman, E., Ernst, E. K., Rooks, J. P., Stapleton, S., & Flamm, B. (2004). Results of the national study of vaginal birth after cesarean in birth centers. Obstet Gynecol, 104(5 Pt 1), 933-942.