You are clearly very worried, OP, so it's good to get advice and support. It's difficult for respondents to get a feel of the situation from small pieces of information - but support is valuable in helping to gain perspective.
Which came first: chicken or egg? His holiday booking or hers? Looking at what is likely from the point of view of human psychology.....
I don't read it as DH booking a husband/wife holiday, with you, to be near to her as she enjoys herself.. (Inventing a work trip he goes on alone, in home country, and actually going alone somewhere with her - would be psychologically credible and typical). But this? No.
Having OW see him with his wife risks putting her off, due to guilt, or alternatively jealousy. You might cuddle him on the beach. She might confront you - which he obviously wouldn't want as he's being secretive with you and denying stuff. What if one of the hen party caught them kissing on one of his sorties without you? That hen friend might come across the sand and tell you when drunk; they might tell you when sober.
Can't see the chicken and egg being that way around.
I can see it that he might have booked his holiday with you already ....and told her ....she might have got jealous and said she wants to be there too in order to satisfy herself that you and he are not a close couple. And he might have said " All right, then, come there too. You'll see that we're not lovely dovey, and it's a functional marriage."
But how would she have contrived a hen party going at just the same time? She couldn't have done.
If she was in that group chalet by the beach, it's unlikely DH would keep drawing your attention to the building.
I think the likely scenario is that it isn't her in the photo. You could, in a parallel universe, try to get a clearer image by means of photo lab technical wizardry, and examine whether the figure really looks that much like her.
The person at your door is neither here nor there. It could be your neighbour has some weird reason for saying that. Did he/she snoop in your garden and invent a cover story in case someone else saw them? These technologically-enabled days, people text or phone - they don't call at the door.
If suspicious, do the usual thing of checking clothing and looking out for changes in personal habits on his part - new clothes/underwear/ personal grooming, etc.
Listen to posters who say you might be over-reacting, as you don't want to sour your marriage unnecessarily. But do be vigilant.
Do some social media checking.