Is someone who suffered from severe depression and refused to eat or drink until they died considered a suicide victim, and is it permissible to pray for them?

If depression affected your relative's mind and prevented him from eating and drinking until he died against his will, he is not considered a suicide. However, if he willingly refrained from eating and drinking to kill himself, he is considered a suicide. In both cases, the funeral prayer is performed over him, because he does not exit Islam; rather, he is considered to have committed a grave sin.