Can I Buy an iPhone13 her in USA but to use in Ecuador

Theoretically it would be fine but she needs to check with her carrier to make sure they can support it. As mentioned, they would need to support eSIM also since there is no way to use a physical SIM card in the 14.

Just noticed you were referring to the 13. The eSIM part is irrelevant but checking to make sure her carrier supports the 13 is still valid.

Since the US model doesn't have a SIM card slot, she would need to use a carrier that supports an eSIM.

Find wireless carriers and worldwide service providers that offer eSIM service - Apple Support

It looks like the only carrier it would work on in Ecuador is Movistar.

https://www.movistar.com.ec/en/movistar-esim

Op asking about an iPhone 13.. not a 14

Kurtosis12 wrote:

Since the US model doesn't have a SIM card slot....

Ooh, good catch!

In that case, you can't buy one, Apple has discontinued the sale of the IPhone 13.