Hey RIKU had offered Sora the choice freely of where he wanted to stay for the holidays. SORA had chosen to stay with Riku. And if SORA had asked to make a trip down to Twilight Town, Riku could have gladly lent him his car, or gone with him if Sora wanted the company.
"No," Riku said honestly. If Sora was going to come right out and ask, Riku was going to be perfectly honest with him. It made for less confusion and miss-communication later. "You know I am uncomfortable with her. But I also know you're not really asking me." Sora had already committed himself to going. If he was actually thinking of not going, he would have told Kairi to give him a minute to think about it. "And I also know that she is an important person to you," and Riku would never insisted that Sora stop seeing someone. "So have fun, and I'll see you later."