Nicknames

[GGilbert]

So your DP works with a female 20 years younger and has a nickname for her
She is in his phone under that nickname
He constantly gives her opinion on things
Drops her name into conversation
To the extent I have to say 'Not interested in what ...... thinks '

I wouldn't like that at all. Would make me think he fancies her if he can't stop talking about her!

I have a nickname most colleagues have known me by (a shortened version of my surname) so I guess it depends partly on what type of nickname it is? I get that many would feel over familiar but some (like mine) were what I was known as by most people and in my industry (PR) it was a very social kind of job so not unusual to have nicknames.

Your OP reads like he has given her the nickname - like a pet name - I wouldn't like that one bit, unless he is the type to do that generally to people.

Giving her opinions: mentionitis!

Ooh yes @StarFriend you've explained it better than me - I have a nickname many colleagues have used (boring, non emotional and a shortening of my surname) but that's very different from a pet name. That's a better way of distinguishing between the two than I explained - so OP is it a nickname or a pet name?

He fancies her

One of my colleagues has nicknames for the entire team, male and female, of all ages. He most definitely does not fancy all of us. I think it probably depends what sort of nickname it is, affectionate pet name or random daft nickname.