You have detected that the person speaking is, from a Brit. Eng. point of view, using very poor grammar.
It should not be:
EVEN IF I WOULD HAVE STOPPED HER, IT WOULD NOT HAVE HELPED.
but as you say:
Even if I had stopped her, it would not have helped.
It was probably an American TV program. There is some debate as to whether their grammar is atrocious at times, or whether such instances are to be regarded as an 'acceptable variant', and even Standard American, since such 'instances' are becoming ubiquitous. Hence, your confusion.
ubiquitous: found everywhere. (A slight exaggeration on my part - but only 'slight'.)
(Please, oh please, avoid 'blog speak' and 'text speak' as in your "coz" and "plz", when writing in the forum. It would be interesting to see, after a time, whether you can still spell those words!) It's bad enough you do not capitalize the "I" - and in fact, seem to eschew capitalization except for your use of emphasis... and then, punctuation doesn't seem to exist for you. You have acquired some bad habits!
Student or Learner