please explain what is wrong with this sentence

It's missing a period/full stop.
Otherwise, the sentence is okay.

I was told that in " He doubts that she will help him " we should replace that with "if/whether"

If you had already been told that the sentence needed to be changed in exactly that way, why did you ask?

I don't agree that it needs to be changed.

I think the sentence is fine. "I doubt that....." is a perfectly correct structure. If you want to use "if/whether" you can rephrase the sentence and say: "He isn't sure if/whether she will help him"

BarbaraPAIf you had already been told that the sentence needed to be changed in exactly that way, why did you ask?

The source is not trustworthy, I also dont think any thing is wrong with the sentence, It had come in an exam

Indeed. It was fine.

Thanks I came across this in the net

a simple rule:

doubt that is used for negative contexts or questions:
There is no doubt that ....., there never was any doubt that.....Is there any doubt that ......?

doubt if (or doubt whether) should be used for everything else:

I doubt if/whether she'll do it. I doubt whether/if he'll be there toda

av abhishieka simple rule

You can use that rule if you wish; it will do no harm. But many speakers use 'doubt that' in both situations.

CJ

av abhishieka simple rule:
doubt that is used for negative contexts or questions:
There is no doubt that ....., there never was any doubt that.....Is there any doubt that ......?
doubt if (or doubt whether) should be used for everything else:
I doubt if/whether she'll do it. I doubt whether/if he'll be there today.

I advise that you avoid do