Hello there.
Here is how things are with Lisa. You were in a relationship with Lisa for over 3 years. You got separated from her due to some unfortunate situations. So, the MC and Lisa are meant to have deep connections. If you accept Debby, from MC's point of view, if I am going to have a relationship with a Girl that you barely know, why would I reject Lisa whom I had a relationship for over 3 years? On top of that, Lisa and Debby will be living close to each other while working together. From MC's perspective, it felt too shitty to date Debby in front of her while rejecting Lisa.