Manjari craves for Harsh's approval. For her Aarohi was the right choice because she's a doctor and Harsh was happy with her for bringing a good rishta for Abhimanyu. When she thought that Abhi also likes Aarohi her happiness knew no bounds.
Then of course everything changed when Abhi expressed his love for Akshu and Manjari had to accept that. When things escalated she didn't do anything because subconsciously I think Harsh's approval is more important for Manjari.
Now she knows Abhi is unhappy but she thinks that getting a girl who loves her son will fix everything. What else do you expect from a woman who has craved for her husband's love and approval for so long? She thinks getting a life partner who loves you is a blessing and that's the quick fix for everything and she shudders at the very thought of one sided love for Abhi since she's been in that hell all her life.
So, she'll go all out for saving the rishta that will make abhi happy in the future. He's not happy right now but Aarohi's love is the balm that will save him. I think this is Manjari's thought process.