Ultimately, I am not sure that it really matters. SA has already said she was last seen by him @ 3:15. So it does not matter if she was out and about at noon anyway.
It matters if they think Hailey may have disappeared between Sun. night and Mon. morning. That possibility has been strongly suggested here several times, so yes, it does matter.
Since SA is the prime suspect, I would say that LE is not taking his statement as the truth. They need a verifiable accounting of when Hailey was last seen, and if this hairdresser's statement can be confirmed, that narrows down the timeline a bit. And removes Billie from being on the suspect list, since she was at work and I'm sure that has been verified easlily enough.