I definately agree with Sophie!!
Ask her to help out!
I'm 16 and I often stay with my boyfriends parents in london. And I'd hate to think for a second I was being a burden on them. I feel guilty having to stay there as it is...
And if there obviously not getting on, why is she staying? I would have probably left.
Plus for future reference, tell your son, if he still doesn't get along with her that she can't stay.
Therefore killing to birds with one stone- Shes not round the house in a strop and your son doesnt have to be fustrated about her being there.
Good luck and I hope the rest of the weekend goes well for you.
xx