Given that we know so little, in fact, all we know is that Christina and friend were seen entering the garage and she was never seen again. It would be useful to get more information on the following:
a) Where exactly was her friend parked, and how close was he to her vehicle?
b) What time did he leave the garage?
c) Did Christina's phone ping anywhere after entering the garage?
d) Was the friend's phone pinging and did it show he went straight home?
Some thoughts: This might not mean anything, but when they were caught on video, they were both walking on the right hand side, with the friend being on the right of Christina. Could this indicate that they were both parked on the right side of the garage?
Lizardqueen observed that the left side seemed mainly reserved for valet parking (would that change at that time of night, or whatever time it was that he parked?) She also mentioned that while there was a possibility that if he was parked on the left side and she was on the right that she might not be visible, but if they were both parked on the right then he should have been able to see her.
What time was he caught on video leaving the garage? Was it a minute or so after they entered or did some time pass? I'm thinking that the video was not clear enough to be able to see if he was on his own or not, or that would have been reported. Have the other vehicles that left around the same time been identified and have the owners been contacted or come forward?
LE have not released any information about whether or not Christina's phone pinged after she was in the garage (not that it would show that she was necessarily with the phone, but it could indicate that something happened in the garage). The friend may have had his phone turned off b/c of the time of night, but Christina would probably have left hers on if she was planning a long drive home.