Not sure if you've ever been in a long term relationship before where sex teters off, but its actually a really hard subject to approach.
You have to consider is it early menopause, is her libido going down, is it depression, is she just not attracted to you anymore.
And asking any one of these questions is bound to start an argument if its true. Girls can be pretty defensive when it comes to the subject and feel like they're being attacked.
His idea to subtly get the mood right is definitely better.