Hi Shahed and welcome,
From your PM I learned that your wife is HbE beta thal, which is common in eastern India and Bangladesh. Often this manifests as thal intermedia but of course this can change during pregnancy, and as is the case with your wife, transfusions can become necessary. Hopefully, once she has the child, she will no longer need transfusions. Have you been tested also? This is important as you need to know if there is a possibility your children will be thal majors.
As far as diet, red meats are high in iron and should be kept to a minimum, if at all. Your wife should avoid citrus fruits like oranges and lemons during meals. These fruits can be eaten but they help iron to be absorbed so they should be avoided during meals and instead taken as snacks. The iron from fruits and vegetables is not as easily absorbed as the iron from meats, so not as much caution needs to be taken. High iron veggies likes the dark green vegetables such as spinach should not be eaten in large amounts but it is OK to have them occasionally. The nutritional value of most fruits outweighs the small amounts of iron that they carry and should not be avoided. If you want to give us a list of foods you want to know about, that would help. Fruits like pomegranate and berries are very good sources of many vitamins and minerals and are also good antioxidants. Your wife would also be advised to take a B complex vitamin, as this helps build blood cells. As Narendra has pointed out, drinking tea with meals helps inhibit iron absorption from food. Milk products will also help prevent iron from being absorbed.
Do not be concerned if she requires regular transfusions during the pregnancy, as this is normal and even some thal minors will require transfusions when pregnant.