It was getting too big in the comments, so I recast this as an answer.
Mention the other claimed proof, and offer your own proof.
I had a similar problem with my thesis. When I tried to publish a paper, the main result was rejected on the basis that it was a well known old result. On examination of the literature I found that a related but incomplete result had been published.
My supervisor said that I should not worry too much because everyone does things their own way. If you honestly did it yourself, the chances are that you will have something new to say.
My paper as a whole was my own work. On the other hand, it took an international flight and visiting the other academics institute to chat with them for a week before they agreed to back off. It was a successful trip as they even became a reviewer for my thesis.
If you did not copy the result, if it was genuinely your own work, then I do not see that you are committing any moral outrage. If the result happens to be well published and you can add nothing new, then the paper should be rejected for lack of originality.
The moral problem would be if you read the person's work, copied the idea, and claimed it yourself.
But, you should definitely note that you became aware of a CLAIM that there is a proof - a claim that you tried to but failed to validate.
At the very least, the result appears not to be well known in the literature. And that is important. Bringing attention to something not well known is useful, as is (potentially) giving a new proof.
The moral problem here is that now that you know that someone claims an earlier proof - as much as you might be tempted - you should not ignore that you now know. You might wish that you never found out. But, that is not for us to decide. What is for us to decide is what shall we do with that information.