who know his bike better than he does if the mud had a lot of sand in it then he could have a problem in his BB or his Back cassette bearings or sand in his chain
first I would give it a good inspection of the chain and then the bearings front and back the resistance is coming from somewhere and if he does not get it clean he will be replacing parts soon
sand is the stuff that they put on sandpaper to make it wear down metal
so check it and clean it
my opinion for what it is worth but you know what they say about opinions ??