Dating or flirting with someone who was your friend’s partner is okay.
Your friend does not owe your life or his life.
You are both free!
She might show some resistance for you dating him but she would get jealous with any girl, so consider yourself free!
You are free and so is he!
This is important!
It is about your love live, and feelings.
It would be a pity if you both like each other but at the same time don’t take action because you feel you are not allowed to.
Remove any feeling of guilt or shame and flirt with him if you want to.
You don’t have to suppress any of what you feel.
Don’t make it heavy though.
Don’t go into serious “dating” conversations, talking about his ex or anything along that line.
Now, it is about you two, not about her anymore!
Keep it light and fun and enjoy the flirt for what it is.
There is no shame and no guilt in expressing openly what is happening.