It sounds like your mom is lonely and doesn't have the tools or knowledge on how to ease that feeling in a healthy way. i.e.: How to Make Healthy Friends.
I also think al-anon would be a good suggestion to your mom, but also maybe something a little more mainstream and less intimidating, like a book club (free, easy, accessible, large age range) or some other shared interest group in her community so she can get out and meet others and make connections.
Also, I personally think the message is just as important as the delivery. My suggestion to you, is that while you want to protect your mother from future suffering, it sounds like she is really trying to tell you that she is suffering right now. Offer her empathy and compassion before you give her any suggestions or telling her that she's making poor choices. Maybe she really just needs someone to listen to her and maybe your dad is giving her the feeling of being heard that she isn't getting from any other source.