It looks like an 8 legged critter to me! One of the 'clover mites' or similiar....there has been quite a few posts about them lately.

By all means, spray them off with a mild,soapy solution.

That's a mite! Particularly if it is the size of the head of a pin. Amazing that you got such a clear photo.

We have lots of tiny red bugs on our wood deck. They seem to appear more when it's really hot out. They're about the size of this . ~ they're not bright red, I don't think they fly, but they do somehow latch onto you if you get too close, then the itching starts! I've even found them crawling on my computer screen now and then, which I assume is because of the heat. Any ideas? We have a dog so we don't use pesticides.

Looks like the mite that destroyed our Paperwhite Birch and many others in the neighborhood years ago. Tell your SIL to keep a good eye on his trees, etc.

Not sure how much damage they could do to already cut lumber, but wouldn't want them there. Sandy

Looks like Spider Mite to me.

Here is a link that might be useful: Spider Mites

Spider mites are smaller than what the OP describes, and they won't be found in large numbers other than on plants.

The 'clover mites' are a common nuisance in the summer time. They are typically described as being on the siding, along window sills, on the deck/patio, and even in the home.