Oops... I totally missed where you posted she has already deleted all the texts.
That's strange, though, and takes quite a bit of her credibility away, IMO. These aren't the days of 2kb phone storage and needing to delete messages all the time anymore. Why would she have deleted them?
Can you delete facebook messages these days? I can't remember being able to permanently delete them there, so maybe you can see if those exist? If they met through a group, then surely there will be some contact on there.
But... I just don't know... strictly due to his lies by massive omission, myself, I think I'd be more suspicious of him than of the woman who came out of nowhere, with specific details and no apparent motive to lie, claiming she'd slept with him.
Here's what I would do in your shoes:
Look back and think really hard about his behaviour these past few weeks.
Did he take any off-schedule showers he wouldn't have otherwise? Did he seem more, or less, affectionate than normal at any time? Did he ever want more, or less, or different, sex to the usual? Was he moody or distant at all? Did he spend unusual amounts of time on his phone, and/or was he trying to keep the screen blocked? Anything like that??
Then, assuming you're still in contact with her, ask her for more proof. If she is genuinely telling this because she thinks it's for your own good, I would think she's be as forthcoming as possible. Ask her what she can do to prove this to you.
If you haven't already discussed all of these specifically, ask her for anything she has like FB messages, deletion records (if possible) or screenshots of clearly obvious deletion, voicemails, any other possible mode of contact. Ask her for all dates and times, and line those up to your schedules.
Perhaps, if you can bring yourself to, ask her to describe him naked, or if he has any distinguishing scars, tattoos, or other characteristics, see if she can remember anything like that.
If she can't come up with anything at all, and everything else has seemed normal with him, then it may be time to block her and write her off as a nutter trying to destroy your relationship.
Then have a very, very serious lecture discussion with your OH about lying by omission and spending time inside the homes of single, drunk women.