What is the ruling on a couple's fasting in the case mentioned in the question?

Intentionally breaking the fast during the day in Ramadan (definition) is one of the gravest sins, and the husband must offer sincere repentance and make up for the days he broke his fast. It is not permissible for the wife to request or enable her husband to embrace or kiss her during the day in Ramadan if that leads to arousal. Your fast for the mentioned day is valid unless you are certain that semen was discharged; if you are certain, then you must make up that day. The discharge of madhy (pre-seminal fluid) does not invalidate the fast, according to the more preponderant opinion.