Hi Prateek ,
There are 3 issues in your question.
1) If you are worried about Mr X business income then Rent paid on residential house will be disallowed irrespective of whether tax audit is applicable or not or whether tds has been or has not been deducted. You can remove the thought of including it in business expense.
2) If you are worried about how Mr X can claim rent as deduction in his return then he cant as it will be application of income rather than an business expense in the hands of Mr X.
3) Whether TDS is to be deducted or not is totally different point because as residential rent is ab intio disallowable you need not bother about once again disallowance by way of Sec 40(a) . But if you dont deduct TDS then you shall be treated as assesse in default and will be liable for penalty.
But for TDS on residential property is matter of debate. As per practice TDS is not deducted but as per 194 I, it quotes 'Any building' so it doesnot make differenciation between Business or residential property.
Thus TDS can be deducted to be on the safer side. Or if it has not been deducted , then relaxation can be taken when the landlord has included this rental income in his return and paid tax on that.