If a girl parties a lot and says she is a horny drunk but that she will not cheat on you can you trust her?

This is just like anything else, either you trust your partner or you don't. Being drunk may cause someone to be more flirtatious but should never be used as an excuse for doing something regretful, such as cheating while in a relationship. If you're even asking whether or not you can trust her then perhaps there are more fundamental issue in your relationship that need to be considered.