Let us look at this incident by stepping into the shoes of the 3
parties mentioned.
1. TS
He caught his buddy going to his gf's house. He got angry, he
whacked
his buddy. Now, he's facing charge of assault?
Q : a) Why did TS think his buddy
is interested in his gf?
b) Were there any previous incidents whereby he saw his buddy
going
to his gf's house?
c) Did his gf complained to him about his buddy harassing
her?
d) Has he confronted his buddy about his perceived interest in his
gf?
2. [u]Buddy
Buddy was caught going to the house of TS's gf. He got whacked.
He
made report to get TS charged.
[i]Q : a) How strong is Buddy's relationship with the TS?
b) Did Buddy got any encouragement from TS's gf such that he
felt
he had more than a fair chance of wooing the girl?
c) Did Buddy felt that TS will not mind his being friends with the
girl?
d) Did Buddy defended himself and returned TS's attacks blow
for
blow?
3. [u]Girl
TS's gf. Witnessed TS whacking his Buddy outside/inside her
home.
She will be the main witness should TS be hauled to court. She
will, for
sure, be told by her parents & family members not to get
involved
especially if the fight ends up as a police case.
[i]Q : a) Does the girl regards TS as her boyfriend or just a
friend?
b) IF, she and TS are an item, why didn't she inform TS about
his
Buddy going to her home (citing reason if it's an innocent
visit)?
c) Did she stop or try to stop the TS from hitting his Buddy?
The answers to the above questions will give us a clearer picture
of what happened.
It is a fact that whoever is the one to FIRST hit another person,
is in the wrong. But if there are no other witnesses, except for
TS's gf, TS will have a very good chance of getting a lesser charge
(disorderly behaviour in public?) if he said his Buddy was the one
to provoked him to defend his honour or whatever. 
I personally feel that MISCOMMUNICATION always cause
MISUNDERSTANDINGS. 
