Something to keep in mind when thinking about going down this path is whether or not you want to be a part of cheating. Because that's what it is. Polyamory in its most literal definition is simply loving more than one, but in reality it entails more than that - it entails the desire for ETHICAL nonmonogamy which means that openness and honesty between all participants is necessary. If you want to be complicit with cheating, that is your choice, but I feel like that kind of defeats the purpose. If he lies to her, what is going to keep him from lying to you in the future?
She may not treat him well, or he may say that just to make you feel like it's okay to be involved with him. Either way, he has made a commitment and promise to her that he is breaking by being with you. If she is really that terrible, why doesn't he leave her? If she isn't able to satisfy his needs, why doesn't he at least give her the opportunity to consent to him seeking it elsewhere? Is it really fair for him to decide what kind of relationship she is going to be in without consulting her at all?
I know some people think it isn't their problem as long as their SO (in your case your MM) isn't cheating on them, but I personally feel like any involvement in the cheating is just as bad as the cheating itself.
It's your decision whether or not you're comfortable with the situation, but before getting in too deep you may want to consider how comfortable you really are with his apparent acceptance of cheating as appropriate behavior in less than satisfactory relationships.