My in-laws who reside in Mexico City have had a change in their holiday plans and I need to buy them a ticket from Tulsa to Dulles and then they will use their original ticket to go back to Mexico City. My question is can I buy the RT from Tulsa to Dulles and just throw away the return since they will be going home on another ticket to Mexico. The RT is much cheaper than buying a one way. Also, if this matters they are notUS citizens.

Yes they can. No need to worry about anything unless they're doing this frequently.

But check prices on southwest to IAD or BWI. They price it by each way, so one-way on them can be cheaper.

Yes, you can use the first part of a ticket and not the return. However, you can not do it the other way around. If you do not show up for the first flight, the 2nd have of the ticket will be voided.

You can also check the fare for an open jaw TUL-WAS-MEX. Then throw away the IAX-MEX return.

As long as the part you are using is the first part, you're cool. No one can force you to show up to use the return half.

Here's a suggestion for good karma: as a courtesy to other travelers, cancel that return with the airline, rather than just tossing the ticket out. It will free up the space for someone who may need it to book the flight.

I have said this many times. If you call and cancel, the airline may have to re-fare you and charge you extra.

Airlines know some passengers will no-show. That's why they oversell flights.

You don't have to do anything. Do so is asking for trouble.