Need more info & pref. Hoof pix, of a variety of angles. Is your mule shod or bare(I hope not shod at that age at least)?
It depends on other factors as to whether the separation is likely to just grow out after soaking. I suspect that advice was because there is infection in the separation, so she wants you to soak in something that will kill it? That is one necessary treatment, as the infection will continue to eat into the laminae & undermine any good work in trimming & management that may otherwise help the hooves heal. BUT the infection didn't cause the separation & killing it won't fix it alone. Infections generally can't get into a hoof until it's weakened from other factors.
It is likely that the horse is, or at some stage has been laminitic, which has weakened the lamellar connections. This is most likely diet related, so ensure she gets a good, low sugar/starch diet & it's fed naturally(little & often). Good, balanced nutrition is also important in growing good hooves.
It is also likely that if the laminae became very 'stretched', if there is flaring, if the horse is shod, that at least part of the problem is that the hooves haven't been well enough trimmed and there is too much pressure on the(or parts of) hoof walls. Generally it's about the walls being too long in relation to the sole plane. This causes leverage forces to tear the hoof wall from the foot at every step. Unloading the walls is a necessary step, to allow healthy growth to come down, without being torn apart.
BTW, especially with mules & donks, they can retain a lot more sole than horses and it can be hard to tell healthy, live stuff from dead that can be exfoliated. Also they tend to grow high, straight heels a lot easier than most horses, and many farriers also believe they should be that way. That means that there may well be a lot more excess wall than is obvious, especially at the heels. High heels also force the animal onto it's toes, so there may be excess stress at that point for those reasons.
So... if she has already trimmed the horse in such a way as to remove the undue pressure from the walls, and you're certain the horse is not currently laminitic, soaking the hoof to kill infection and keeping the walls relieved with frequent good trimming should allow the separation to grow out. If what you/she means by 'take the hoof wall out' means that she hasn't yet unloaded it, &/or that she plans to 'resection' the wall, then I disagree with her approach & don't think that's a good idea.