After the death the widow become absolute owner of the flat. If a will arisen the widow can challenge the same before the court. From your query the probation will is not allowed by the court due to the absence of necessary party(widow of the deceased person). So adding the widow as party array no one can act as per will.
The onus to prove the will is on the propounder and in the absence of suspicious circumstances surrounding the execution of the will, proof of testamentary capacity and proof of the signature of the testator, as required by law, need be sufficient to discharge the onus. Where there are suspicious circumstances, the onus would again be on the propounder to explain them to the satisfaction of the court before the will can be accepted as genuine. Proof in either case cannot be mathematically precise and certain and should be one of satisfaction of a prudent mind in such matters. In case the person contesting the will alleges undue influence, fraud or coercion, the onus will be on him to prove the same. As to what are suspicious circumstances have to be judged in the facts and circumstances of each particular case.