Upvote:5
When you enter the US, whether on a visa waiver or visa, you'll likely be asked to show proof that you intend to leave and not try to stay. For you any valid ticket out of the US should do, including to Mexico. For your friend however, a flight that ends in Mexico won't do and I recommend they get an additional ticket out of Mexico to be safe.
You should be aware though, that this doesn't reset the VWP clock for your friend. In case they return to the US their stay in Mexico will be counted in their 90 days.
Upvote:8
In general, citizens of Chile do not require a visa to enter the US (and can instead enter via VWP with an approved ESTA), however in the comments you've stated that you have a Visa, whilst your travelling companion from Germany does not and will use ESTA. Thus there's 2 different answers...
With a Visa - There is officially no need for a return/onward ticket when entering the US with a visa. However even with a visa, your entry to the US is at the discretion of the border officials. If they believe you intend to over-stay your entry then they can deny your entry. Having an onward ticket will reduce the chances of this occurring.
Without a Visa - When entering under the VWP program there is a requirement to have an onward or return ticket. If that ticket is to either Canada/Mexico (or several countries in the Caribbean), then you ALSO need a ticket onward from that country to another country that does not border the US (unless you are a resident there).
So for you, a ticket is not required but may be advised - and Mexico will be fine. For your companion, a ticket is required - and Mexico will NOT be suitable without also having a ticket from Mexico to somewhere else.