I'm thinking of starting a relationship with a girl my friend recently broke up with
Do you think your friend would be upset?
I don't think they would mind.
You should ask your friend first. If they recently broke up then they might still have feelings for each other.
They should be over it by now.
They might be over it and not care. But you won't know if you don't ask. If you don't ask before you do it, you could ruin your friendship.