Please explain "if is'' here

Of course it is wrong. The sentence should read:

The only thing that would have made this evening more perfect is if Peter had been here.

The only thing (subject)
that would have made this evening more perfect (relative clause to qualify the subject)
is (verb)
if Peter had been here (a noun clause serving as a subject complement)

i copied this from a film script. So it's wrong grammatically.

Yes, it should.

Should your sentence be written as:
"The only thing that would have made this evening more perfect is if Peter had been here." ??