The fibrocartilaginous part of the name just means that is the material of the bridge between bones is not ossified.

Here is the drill:

Children appear with symptoms, adults tend to be asymtomatic, so I assume you are talking about a child in school.

The first step is to set goals:

The nonsurgical treatment should be attempted first,
since the surgical may not restore full participation in sports and physical activities.

With that understood:

1. relieve pain

2. return to school and participation in all but sports activities

3. stop all sports and physical activities (PE)

4. return to sports activities

5. decrease calf atrophy

Relieve Pain-

Nonsteroidal anti-inflamatory drugs (e.g. Ibuprofen)
heat
paraffin baths
warm soaks
whirlpool

if pain is severe- below the knee cast x 4 to 6 weeks

alone or combined with

peroneal nerve block
steroid/anesthetic injection in sinus tarsi

When the cast is removed-

Orthoditic devices and modified shoes
with progressive adjustments of 1/8 to 3/8 of an inch (medial wedge)
physical therapy
range of motion exercises

I am drawing this material from the podiatrist publication below.
The podiatrist is the equivalent of an orthopedic surgeon who specializes in the hand (in this case feet), only they are a separate doctorate altogether.
They are more highly versed on the tiny variations than an orthopedic surgeon (MD or DO), BUT, and this is a big BUT, you would do well to find one who does a great deal of this particular treatment- flatfeet.

*****************To answer your question on success rate *******************

30 to 90 % respond to the non-operative conservative treatment

Most authors on the subject agree that this should be done before resorting to surgical treatment, particularly since you state the bar (coalition) to be soft rather than bone (fibrocatilaginous).

Surgical Repair-

You WILL correct the condition.

But you may, as stated above, not achieve the full restoration of activities enjoyed before.

Let this be your introduction as you educate yourself to seek specific doctors care.