@LeahYoga You ironically appear to be too embarrassed yourself to come back and reply to any of the other responders who were kind enough to use their time and effort to answer your question. I am reasonably sure that most of them replied for the sake of your dear son, and his dear friend, to try to get you to understand just how embarrassing it is for teenagers - especially those of 16 years and under (as the older ones would probably just wet themselves laughing at how uptight their poor mate's mum is!) - to have to deal with all sorts of emotions that are suddenly making themselves very uncomfortably, and confusingly, known. Were you never a teenager OP, or have you just buried those awkward and often embarrassing, but sometimes also amazing, teenage years, into the dark recesses of your mind?
So who, or what, is more important to you OP, the feelings of a young person unintentially having a quite severe bowel reaction in your toilet bowl, with maybe some of it even getting on to your toilet seat (isn't it a great pity that whoever invented toilet seats didn't make them easy to clean), which he hurriedly left without checking, as his embarrassment almost certainly made him vacate the bathroom as quickly as possible; or, is the fact, that to you, it seems much more likely that the young lad purposely disrespected both your bathroom, and you, and therefore, obviously and deliberately, left the unfortunate remnants of what was probably explosive diarrhoea, for you - who he may have previously thought must be a compassionate and loving mother, as your dear son is so very nice - to clean up?
My biggest worry now is that your - presumably and hopefully 'dear' - son, may never trust you to have his back when/if something actually important happens in his life. In fact, you may have, very sadly, already blown that trust, if you were ever lucky enough to have previously earned it. How can he trust you with his innermost feelings and worries, when he can't trust your reactions to the most minor of issues? Please OP, make some - believable - excuse to your DS, to his Dfriend, and to his Dfriend's mum, as to why you reacted in such an over-the-top and unreasonable way. That really is the only way that I think you have a chance to come out of this whole farce, reasonably un-scared. Of course, other Mumsnetters will have probably got much better suggestions for your way forward from this, if you indeed do want one?