Assume that the bet paced in between oscar and vinny is legal in the state in which they live. what is maria (or the fresh air fund's) relationship to the contract

I believe Maria is a third party beneficiary.

Third party beneficiary refers to the group of individuals that have the right to sue for a breach of contract even though they're not directly the original active party to the contract (because they either directly or indirectly affected by the outcome of the agreement)