Assalamu Alaikum - I’m unsure how to handle this situation

Assalamu Alaikum. I need some advice because I’m really confused and worried. My dad passed away nine months ago. My mother is very religious and has always taught me to avoid mixing with non-mahram men. She never allows me to have male friends. Lately I discovered she has been messaging a man on WhatsApp who is saved under a girl’s name. He is actually my sister’s former math teacher. Their messages aren’t overtly inappropriate, but he told her he misses her and she said she likes getting his messages. She also mentioned she feels she has “support” in this town because of him. He asked for her Snapchat and my mother suggested meeting at a restaurant sometime. This is confusing because my mum is a pious, loyal woman who stood by my father through his illness. I don’t want to accuse her without reason, but I’m afraid this man could take advantage of her or cause gossip. I don’t know the best Islamic way to approach this without hurting her or creating fitna. How should I handle it? I tried talking to her about remarriage before and she said she wasn’t interested and that she wasn’t speaking to anyone. After our conversation she deleted the chats with that person, which makes me think she’s hiding something. Please give me gentle, practical advice on how to protect her dignity and our family’s honor while addressing any potentially harmful situation. JazakAllah khair.