Wash it off. If there's mud and grit under there it will almost certainly rub, whereas if it's just wet there's less chance of it rubbing... IMO anyway.
lucky you if its just the girth area! my boy has turned into a total mud monster inspite of having a rug that covers from ears to tail he manages to lift his skirts when he rolls and work clay into rastafarian style clods all over his head, ears, belly and top of his front and hind legs. He's a sensitive TB too which makes getting it off before riding a total nightmare, all suggestions welcome. he is not clipped this year as I am going to give him a break after xmas while I have knee surgery and at the moment can only ride 3-4 times a week so he's living out with a little orange playmate.
Nothing worse than wet mud is there, its not like you can just brush it off.
I'd of got some warm water and a bit of shampers and given it a wash and then get it as try as you could with some towels and then ride, so long as its clean...damp hair...no difference to riding once they hav sweated up under their girth area, again sponge off and dry afterwards...works both ways I guess.