Ref to Section 163 that clearly indicates that the agent or the employee of a non-resident are responsible for compliance of the non-resident for Income Tax Act 1961.
In your case you are not knowing whether your brother consented to NRI using your address. Whether it was consented due to an ongoing friendship, business or any relationship from which he derived any mileage. In your case, as you had not consented to giving the address, what you had to do was at worst write to AO that person is not known to youand there is no such person and NEVER informed your friend of the notice. Informing your friend regd. the notice has caused problem to you as he went and represented in reply to the notice. Had you not informed and written to the department of the non existence of the person, they would have just continued having an unserved notice and the onus would have been on them to find the person. They would have attacked the mutual fund company to find out the basis of the address being given and would have informed you accordingly if there would have been any evidence of your brother allowing use of addresss. If your brother may have given any consent in writing that would have also surfaced up and you would have had the plea of having no knowledge of the particular person as your brother is no more.
Now the AO would possibly be having one correctly served notice in record and that to on your address. With a locally available address that he knows will inform the overseas person, he will continue process locally and he has the powers to rightly or wrongly attach your property to recover dues of your friend. You will have to work hard on getting the attachment vacated but right now if you do not put anything on record, later when you approach the court to have attachment vacated, they will ask you where you were all the time while the process was in progress. You should have some evidence of having communicated the fact to the government.
This is the very common problem. People allow friends going abroad use of address without understanding repurcussions due to gaining social or business mileage. When such problems happen, they do not want the friend to suffer also because of the relationship. In such matters, you cannot think of protecting the friend also and at the same time being left unharmed. The AO / TRO can rightly assume that the assessee is having some interest by ownership or tenancy in the premises on record and you will have to prove otherwise and that is really not going to be easy for you.
As a trader I have knowledge of such matters because I have seen many people get into similar problems. The CAs in this forum would be very professional in their work and no professional would allow the matter to go out of hand the way your friend's matter has gone out of hand to such an extent. In fact they avoid taking cases of such persons who will not pay suitably for professional help to get matters sorted out. By my experience,people like your friend try to get as much work done for very little from professionals.