Maybe she’s thinking of doin’ him in to collect on the insurance, and is just laying some groundwork? 
(Sorry - first think that entered my mind … been watching too many “Forensic Detectives” …?)
Maybe her boyfriend is the one with low self-esteem, and she’s trying to boost his ego a bit.
If she is just a casual aquaintance, not a close friend, you may not know her well enough to determine whether or not her gushing is hiding a problem, or if it’s just what she does.
I’ve seen a lot of people gush during the first heady months of a relationship. Then they get over the initial infatuation and tone down the commentary, but settle down into perfectly happy relationships.
She could be picking up on the fact that some of her friends see a problem with this relationship.
Or she could be totally in love.
If she’s been in the relationship for more than 4 years, something must be going right. I don’t gush about my husband, but I do often say nice things about him to other people. Partly it’s because he’s pretty quiet and modest in real life. (On message boards, though, it’s a different story. 
)
I’d be a lot more concerned if she was always critical of her boyfriend in public. I’m a big believer in the “praise loudly, complain quietly” school.
For other people, maybe, but she’s one of those who refuses to give up and tries to make a relationship work when she really should have called it quits and gone on to something else.
Her ex-boyfriend cheated on her the whole time, but she still tried to make the relationship work. Sure, give him a second chance, but he had a string of like 6, 7 affairs during the whole relationship, and she still tried to hang on until she got hurt enough to give up. 
Nah, that’s putting the cart before the horse. We didn’t start thinking of a problem until she started to become progressively more gushing.
That may be possible, but with the real person or just an image of that person she made up? We also know her bf some, to us, he’s a distinctly different person than the one she’s telling us about.
As I mentioned above, she has been hurt deeply before, and we don’t want to see that happen to her again.
Urban Ranger
“she has been in this relation for more than 4 years”
“Her ex-boyfriend cheated on her the whole time, but she still tried to make the relationship work. Sure, give him a second chance, but he had a string of like 6, 7 affairs during the whole relationship”
“She is a causal acquaintance of mine”
“How I wish she is my SO, but unfortunately she’s not.”
Who has a problem here?
It wouldn’t even cross my mind to say anything disparaging about a member of my family in a conversation at work. When I told people at work that I was getting divorced one of the women said “I’ve never heard you say one negative thing about your wife, not even a joke.”
Not necessarily negative, it could be something like, “He’s learning cooking, and he’s doing better all the time :).” instead of “He’s a god of cooking. ::swoon::”