Is his marriage to his divorced wife, who had married another man and been divorced by him, valid, or does it fall under the forbidden marriage of al-muhallil and al-muhallal lahu (one who makes lawful and one for whom it is made lawful), given that she had told her second husband, before his divorce of her, that she did not love him and wanted to return to her first husband?

"Marriage with the intention of tahlil (making a thrice-divorced woman lawful for her first husband) is forbidden and a major sin, and it is not valid. If the second husband intends tahlil, then the marriage is forbidden and void. However, if the intention of tahlil is from the wife alone, scholars have differed. The correct view is that this invalidates the contract in her regard if it is accompanied by a forbidden action, such as requesting divorce or khul'. As for mere intention, it does not invalidate the contract."