Sofie,
The only time thal minors would have any problem with excess iron absorption would be if the Hb level was very low, <8-9 for long periods. It is not normal for minors to have such low Hb levels, so iron is seldom cautioned in dietary decisions. Supplements should be avoided unless iron deficiency has been demonstrated to coincide with the thal minor. Even then, iron supplementation should be on a temporary basis only. If one did have to avoid dietary iron, the heme iron of red meat is the most easiest absorbed and would be best reduced.
I find that avoiding empty calories, processed foods and junk foods is most important for thal minors. A nutritious diet will help the body produce and maintain healthy red blood cells. I suggest a diet high in antioxidants to counter the oxidative stress caused by the higher than normal turnover of red blood cells in thal.