Based on what little I have been able to find out, a teenager coming from Guatemala likely with no resources, no connections, and with some form of mental disability would probably struggle to make any decisions beyond perceived survival. Working in a food truck...was this family, or someone who hired her? Maybe a sponsor who got her across the border in exchange for her working? Did she work in the kitchen part, or interface with customers? Where did she sleep? Is she here with family or friends or all alone? There are resources for undocumented immigrants in Texas, including health care, but I can imagine that fear of being sent back to Guatemala and the violence, corruption, and poverty there might cause some if not many to avoid accessing any services that might bring authorities. Is her boyfriend her age, or an older man? Whoever it is, is he taking advantage of her, or does he truly care for her? It sounds like she make depend on him for survival, and either she thought, or knows, she would lose that with the birth.
The only positive in all this is that baby has a chance, thanks to the right people passing by at the right time.