Allow me to expand on what I mean. The way I see it, his partner has been stealing from the family bank account. Cheating and stealing are both breaches of trust with potentially serious consequences. But I don't mean to imply that they are exactly the same.
Given the seriousness of these situations, once it happens you have to consider whether you will stay with this person. If you stay with them, how can your partner regain your trust and prevent a relapse? Are you willing to stay in the relationship if this happens again? These are difficult questions that only OP can answer; in other words, both leaving and staying are justified (as is the case when your partner cheats on you).