Evil is seducing...
Evil is alluring...
Evil is... irresistable... 
Regardless, Let me analysis and sort you out bah 
Accepting invitation to your best friend's boyfriend is really
nothing -just that the *CRITICAL ISSUE* is your temptation and your
own personal craving of him. Anyone could have dinner with anyone's
bf/gf, with a clear idea that the dinner is just a dinner between
friends - you think beyond that - you are gone. 
You said gone beyond that - I presume something, sexual, must have
happen, thats why you felt so gulity over it. I never knew of the
situation between the three of you, but allow me to ask some
question here, so that you could analysis yourself. You don't have
to answer me.
I) Relationship between your friendship and bf is good? You *sure*
he is not using you as an excuse to break with her later?
II) He does that to do, he could do that to any *other* girl.
III) Prior before this, I presume he had send you mixed signal for
you to fall into love - he succeeded in this attempt the first once
- he knows he will succeeded in all attempt.
From the III point, what could you derive? He SENSE your weakness -
which is the liking for him. 
Analogy
'He tested the beast - the beast moves back, but never bites. He
knows the beast is afraid; he advance and the beast retreats...
until it is cornered and surrenders itself.'
What's the beast? - Your feeling and liking for him. Its prefectly
alright to to like or love anyone you want - but it is essential to
learn and realise which liking or love could destroy or harm you
instead. He is devious and clever by this 'First Strike' strategy
which made you go instantly 'blind' to your doings.
What do I mean by 'First Strike'? Woman being more emotional
creatures goes blinded when quick advances is being made with
overflowing emotions. You don't know what you are doing because you
can't 'see' momentarily.
Now you regret, you have to stop all these from happening
again.
*DO NOT*
... See him or let him see you - you need this break to clear from
the spell he had cast onto you. Rather to make mistake again, take
this time to recover and do nothing - doing nothing and avoid will
at least stop your emotions from overflowing again.
... call him or answer his call. Voices stimulates the mind and
basically does the same thing.
... ICQ/IRC/SMS Virtual words are collected in your unconscious -
which will affect you without you realising.
Remember: TWO WRONG DOES NOT MAKE ONE RIGHT.
Prays
