You could ask to have the ticketed booking split. I typically would just leave things as already booked unless the travelers are to end up flying on separate flights for whatever reason instead of flying together as originally booked.

For customs, a travel party may at times be considered a travel party together even if on split or separately ticketed reservations. Even with separately booked reservations, the IP address, payment, ticket numbers and such on bookings could give the appearance of traveling together.

My general suggestion for such circumstances is to be forward and straight with the truth when talking to CBP if it comes up but not to worry. This kind of thing is anything but uncommon, especially if you are both planning to fly out of the continent together on time for there not to be an overstay for the foreign travel companion and have no interest in seeking a change of immigration status on or after arrival to the US.

Enjoy the trip, and don’t worry either way. If you would feel better about splitting the reservation and airline frequent flyer program status benefits aren’t part of the consideration (with regard to seat assignments, checked luggage), get it done. If you don’t feel better about splitting the reservation, just leave it as is.

To split the reservation, you should call the airline and/or travel agent to do it. They should be able to split the reservation without having to cancel the tickets and reprice the journey for a new booking.