StSanta,
The first thing your friend needs to realize is that sex is not what holds a marriage together. It is just icing on the cake. If sex is all there is between them the marriage won't last very long.
I have been blessed with a life partner of 37 years (this comming Dec.) who first of all was my best friend. We do things together and enjoy life as it comes...together. We have always maintained the utmost respect and regard for each other and we never part without having first made sure all questions are answered.
Sex is great, but I think it is highly overrated. It should never be the foundation on which a marriage is based. Time takes it's toll on all of us and "frequency" does diminish but if the foundation of a marriage is set in the concrete of friendship, mutual respect, and admiration it will last forever.