Hi MsBehave
Have you spoken to your mum about this or is this just a vibe you have picked up from her when this man is around?
I would definitely speak to your mum and find out exactly how she's feeling. I have to assume that she doesn't know how to approach the situation with your dad, or she's frightened of his response or being ridiculed. It may also be a good idea, if your mum isn't sure how to word it, for you to have a chat with your dad if you have a good enough relationship with him. As your dad's his friend, it's ultimately up to him to do something about this (or choose not to). Maybe your dad doesn't realise that this intrusion is affecting your mum so much.
And to be fair, if it was my parents, I would not keep my nose out. I would also be a concerned daughter, looking out for the interests of my parents. If they asked me not to get involved, then fair enough I'd respect that. But until that point I'd do what I could to help out in a difficult situation.
Although... if I thought that this man was royally screwing up my parent's relationship, I'd probably have a very direct word with him myself, i.e. tell him to back off and give them space, find himself some hobbies of his own. Again I have a great, honest relationship with my parents, so it depends on your relationship with them. He himself perhaps doesn't realise what he's doing and I certainly wouldn't give a damn if I upset him or he never spoke to me again, and if it made him wake up and stop being so needy, it would be worth it, for his sake and your parents'.