With the recent crackdown in enforcement of the dining plan, if one room purchases the dining plan and the other room does not (make sure you have each room on a separate reservation), the key cards for the DP room will say, for example, 2A. When you go to sit down for dinner at a TS restaurant, your server will ask you if you are on the DP or not. If you say yes, you will only be able to purchase the number of meals as are indicated on your card (2A card, 2A meals max at a sitting).
So there is no way that you will be able to tell your server "we want to use 2 of our TS credits to purchase dinner for our friends." This is not allowed and really has never been, but in the past there were loopholes. Those have been closed and reports are that CMs are closely monitored on usage and card details.
However, you can use the plan and the other couple can run a cash receipt. If you choose to pay for them using your own cash, or charge it to your room, you will have no problem doing so...just let the server know what you want to do.