How can I prove it?

I was assualted by (at time) boyfriends friends, while boyfriend stood back and watched. This was planned while I had no witnesses and he had approx. 8, therefor I cannot PROVE anything. However, I recently came across recordings of his daughter who has stated to me that he wanted both his daughters to "beat my butt" about 1 1/2 years ago. Also found other recordings of him stating that I "wanted" him to hit me but he is too smart to do that in regards to our size difference. Do these recordings hold any "proof" in the fact that he did plan the assault?

You can certainly ask that a court look at those as evidence of his character etc.