Wouldn't bother me.
DP has a particular artist that she loves, it's not her usual type of music at all. And I know why she loves this artist, it's because it reminds her of a particular time in her life, with a particular bloke (I know this because she told me). It reminds her of a time of her life that she really enjoyed, that she can't recapture.
And that doesn't make me jealous, because I have memories like that too about ex girlfriends, and sometimes they're triggered by music, or smells, or being in a specific place. And me having those memories doesn't mean that I'm unhappy with DP, or that I wish that I was still with any of those women. It just means that I'm reminded of a time when life was good. Life is good now as well, but it's a different kind of good to then, so sometimes I get nostalgic about then.
I know that sounds confused, I've been in the pub since 11am watching the rugby so I'm not being particularly eloquent.
What I'm saying is that DP hears the song and remembers being happy. The boyfriend was a big part of the reason why she was happy at the time, but that's not why the song makes her happy. The song makes her happy because she was happy when she found the song. In the end, the boyfriend didn't make her happy (he punched her in the face) but at the time he did, and so the song still does.