I have never heard of a case like that. However, it depends where you are from i.e scotland or england etc as the laws in each country vary. i know that in Scotland only a spouse can claim rights (sucession (scotland) act 1964 and cohabitants may make an application to the court for a payment from deceased's estate (s29 family (scotland) act 2006 but any payment is only at the courts discretion and given that it was his mistress, it seems unlikely that they lived together as if they were husband and wife. so based on the succession in scots law, no, but perhaps it is slightly varied in england.