Firstly, never, ever let her stay there again. Secondly, I'd very discreetly speak to the other Mums of the girls who also stayed to A) find out if their DDs also felt uncomfortable; B) to make sure they know the possible risk of letting their DDs stay again in future and C) to rule out the possibility that he may have used the sleepover to actually approach or abuse one of the other girls.
Agree also with MissyMoo to ring NSPCC, it just feels like you should tell someone official about this but I don't know who or how.
Lastly I would discuss with your daughter again, make sure she knows that if there is (god forbid) any more to the incident that she has kept secret, that she can confide in you (it's horrible to say but what if she was testing the waters to see your reaction and something else did happen?)
I don't want to picture a worst case scenario but am just trying to put myself in her shoes, as a past victim of abuse myself. However the fact that she has confided in you in the first place means she obviously trusts and loves you very much. Please make sure she knows you believe her and that she can confide in you - my Mum simply brushed away what I told her and made me feel like I was over reacting.
Yes definitely take professional advice. By all means say that you accept that 10yo girls on a sleepover are likely to be excited and possibly winding eachother up over something trivial, but that your dd's reaction is causing to sufficient concern to ask for advice.
I am deliberately leaving out some details here... What would your gut reaction be if your 10/11 year old daughter came home from a sleepover and told you that the girls had noticed the father (who has 10yr old daughter) watching them getting changed, and that he made them feel very uncomfortable? What action, if any, would you take? What would your thoughts/ concerns be?