Does the intense feeling of waswās (whispers/obsessive thoughts) during prayer, which leads to abandoning it, and connecting that with a bad ending (sū' al-khātimah) and seeing specific dreams, signify a bad ending, or is there a solution to this prayer problem?

The summary of the fatwa is that what the sister is experiencing is waswas (whispers from Satan), and its cure is to disregard it, seek refuge in Allah, recite the Two Protectors (Surat Al-Falaq and Surat An-Nas), engage in frequent remembrance of Allah, and maintain ritual purity without excess or negligence.

She should also repent to Allah if she has committed a sin, and continue in obedience to Allah if she has not committed a sin.

As for the dream she mentioned, this website does not specialize in its interpretation, and it is not an indication of a bad ending. It could be from Satan to grieve her. Therefore, she should fear Allah and have a good opinion of Him.