Should my mom trust her nephew who murdered someone?

my mom has recently been talking to her nephew who is in jail right now for murdering someone when he was in a gang 10 years ago when he was 17 years old. he claims that he became a christian so my mom always talks to him and he asks her for money and he asked her for a wedding ring for his girlfriend, but my sister my dad and I dont trust him at all, because what he says about being a christian and all that stuff all sounds soooo fake, but of course my mom believes him even though its quite obvious its all a lie. my older sister who is 19 told me that she remembers that when she was 4 years old my moms nephew(timmy) tried stealing her car keys when my mom wasnt paying attention but she told him not to and he told her to be quiet. but anyways i just wanted to know what to tell my mom so that she'll know that he's lying and he's just using her.