Those are actually three different options, at least in the currently accepted view of things:
A. Corticosteroids (pred) induced remission followed by thiopurine (6mp/aza) treatment: Still used in low risk patients who have little in terms of symptoms, and who do not fall into the high risk Crohn's group. High risk Crohn's elements are diagnosed below age 30, fistulas or abscesses, broad spread of ulcers throughout the intestine, continued problems for a longer time, history of surgery, narrowings in the smaller intestine (Strictures) etc. The two elements of high risk from your description would be the prevalence of ulcerations and her young age. So that seems a borderline case if she really is doing fine with little to no symptoms (that is no diarrhea, no bloating, no pain, no fever, no decrease in body weight, no vitamin or mineral deficiencies due to malabsorption etc.)
B. Humira induced remission and Humira mono therapy going forward: Used in high risk patients and/or patients which do not respond to corticosteroids or do not tolerate 6mp/aza.
C. Humira/6mp combination therapy long term: Used in high risk patients and patients which do not respond to corticosteroids.
According to the most often quoted study of biologics mono therapy vs. thiopurine mono therapy vs. combination therapy, combination therapy statistically is most effective, with biologics mono therapy in second place and thiopurine mono therapy in third place:
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After 26 weeks of treatment, patients getting the combination had a 57% chance of disease remission, compared to 44% of those getting Remicade alone and to 30% of those getting azathioprine alone. Similar results were seen after 50 weeks of treatment.
The combination treatment worked even better in patients with colonoscopy-confirmed disease and blood-test evidence of inflammation. Among these patients, 69% achieved remission with the combination treatment compared to 57% of those on Remicade alone and to 28% of those on azathioprine alone.
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http://www.webmd.com/ibd-crohns-dis...414/combination-therapy-treats-crohns-disease
To make a long story short, as far as I understand it, the reason why 6mp/aza (6mp and azathioprine are the same effective drug, aza just converts to 6mp in the body) as a long term mono therapy is still used over e.g. humira along or humira and 6mp, is because we still don't have that much long term data on biologics such as humira and there is a slightly increased Tb risk for humira as well as some other side effects that do not exist of 6mp.
One way to go is to start with pred, taper off, see if there is clinical remission and try to keep her in long term remission with 6mp. This is the statistically harder route and less likely to succeed.
The other way is the one more commonly used these days, that is humira long term (no 6mp), which is more effective than 6mp alone.
Lastly, this is where doctors in various countries have different opinions on, 6mp and humira long-term, together. We don't know what the long term side effects of combination therapy are, there is just is not much in terms of data. We know that short term, the serious infection risk is not higher than for humira alone or 6mp alone, rather the above quoted study even shows it is lower. This is by far the most effective way we have today to induce long-term remission in Crohn's patients, but also the one way forward with the biggest question mark in terms of what it means for increased risks from such therapy.