They're not even 20 years old and you have no idea about their families and traditions.

OP literally posted in his, well, OP, that his mother already wants the girl to marry her current boyfriend. By definition, her mother's already "picked out the guy she wants her daughter to marry", and, thus, could be considered an arranged marriage.

OP also said that the girl's mother is Mexican. Catholicism is the dominant religion in Mexico, representing about 82.7% of the total population as of 2010. In the 1970s, the generation that this girl's mother is likely from, 96% of Mexicans were Catholic. In absolute terms, Mexico has the world's second largest number of Catholics, surpassed only by Brazil. It's reasonable to assume that this girl's mother, and her family, are most likely Catholics.

Likewise, on religion, according to Pew Research, Mexicans are more Catholic and often more traditional than Mexican Americans. OP did not specify whether OP's mother was Mexican or Mexican-American, but it seems likely, based on the whole 'arranged marriage' deal, that she is socially conservative and religious. Her mother pressuring her to "marry her boyfriend for his money so he can take care of her" is also definitely a socially conservative value.

Likewise, around 44% of Mexicans believe that the Church ruling prohibiting divorce shouldn't be changed. By the girl's mother wanting to marry her boyfriend, especially so that "he can take care of her", the implication is there that this girl would be marrying her current boyfriend for life.

There also seems to be clear parallels between the Mexican mother and (Mexican-American?) daughter here, in terms of a divide in values and the daughter being more liberal, and /r/relationships posts on the same divide between Indian families and their Indian-American children. There is also the same attitude of, "despite being raised in a more socially liberal country, the Indian-American daughter is still expected to enter an arranged marriage with a spouse of her parents' choosing, honoring traditional Indian values".

However, this girl is looking to cheat on her boyfriend with OP. Any form of cheating means that she doesn't love her current boyfriend, and by definition, she's trying to use OP to fulfill her sexual needs.

That's not love. That's a shitty form of manipulation she's using in order to have both ways, since she's refusing to break up with her boyfriend "because her mom will disown her",