Well, for what it's worth, here's my view. He says he's not attracted to her anymore. You say she hasn't really changed all that much.
This leads me to believe it is more psychological. Translated, that means she just isn't as much fun to be around as before. If she was, and her looks hasn't changed much, he would still feel the same as before, and would thoroughly enjoy her (Even intimately).
Our opinions of others shape how we see them, both physically and psychologically. So, either she has changed in some ways (behaviorally), or he has changed in some ways (Perhaps not happy inside for other reasons), or perhaps both, and that emotional feeling towards her has been altered, which affects how he sees her physically as well. Our mind discounts physical appearance, to some degree, when we find that person attractive in other ways.
Even though they still do things together, he may be experiencing it as an effort to try and salvage something he is not necessarily desiring, and so it isn't really the fun and enjoyable experience it should be.
If this assessment is true, then he would need to figure out what has changed, and be willing to look introspectively at himself as well, to see what might have contributed to the way he sees her now. Maybe it can be altered. Whatever the case, I don't think he should place the burden on her to make him happy. That needs to come from within, and then he brings that happiness to the relationship to add some flame back to the fire.
That'll cost you a nickel, but I'm willing to negotiate.