If his best friend is going to be pissed of with him, and he goes and sleeps with this girl - what is going to happen between him and his best friend - will they still be best friends?
To say 'your best friend didn't have a relationship and has no claim to the girl' is both true, and misses the point (the poster is worried about his friendship with his best friend, or he wouldn't have posted)
As we certainly don't have enough information to work out what is the best path through this - any advice is fraught with danger....that leaves questions.
The questions should be obvious :
- how will your friend react
- what is important to you
- how will you choose to resolve this
Most people will just do...and what they do shows the middle question rather clearly.