Is there anyway to make people see how stupid their decisions are?

So my nan in her 70s met a guy 3 years older than me, online a month ago. Has only visited him once. Then said he's going to move in with her next month and that we (several members of the family, her son, her sisters), don't know what we're talking about when we say he's probably trying to scam you. As he brought her stuff, and he's a super nice guy, she met his parents and his sister and he brought gifts for the kids (my siblings both teens and my little one) so there's no possible way he's scamming her.

She also has a bad habit of jumping heart first into relationships and has ended up burnt - sometimes in normal ways, other times in terrible ways, dozens of times. It's part of the reason I am the way I am, after hearing about 10+ relationships she's had it made me want to be smarter.

Is there anyway to break the love sick spell, I fear this is going to end with her being bankrupt and even if by some chance this is genuine, moving in together after chatting for two months and only meeting once just spells disaster.
Is there anyway to make people see how stupid their decisions are?
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