Is it permissible for the uncle of a wife whose husband has passed away to see her during her 'iddah, or should he limit himself to greeting her in the presence of her children?

May Allah have mercy on your father and forgive him. Your foreign paternal uncle treats your mother as a non-mahram (a woman he can marry). If she is an old woman for whom there is no desire, it is permissible for him to greet her and look at her without being in seclusion with her. If she is a woman for whom there is desire, he must be cautious in his dealings with her. It is permissible to greet her without shaking hands, provided that there is no fear of temptation. She must observe hijab in his presence, and he is not permitted to look at her at all, according to the more preponderant opinion. This ruling applies to any non-mahram woman generally.