I don't think two people would necessarily have to be involved, but it makes more sense logistically if this were the case. If it was an alleged Marine boyfriend who was involved, Erin and this person would've seemingly had to have left the base in her car (if she did not park it there herself). The perp would more than likely be on camera if this is the case. On the other hand, if there was collusion - say, Heather Elvis style - then it makes it much easier to leave the base without being directly tied to Erin. In other words, if he and his wife (or someone else) were involved, there is plausible deniability if they were seen leaving that AM in a different car and returning together later.
Basically, in my opinion, if this alleged boyfriend and Erin left the base in her car, he'd be in jail right now.