Man spending lots of time alone with best friend's wife. Aibu to think this odd?

You say the husband knows that his friend visits. So there's not a concern on his part about what is going on whilst he's out with their child.
Perhaps they have an open marriage. Perhaps wife and friend are cheating. Perhaps it's all innocent.
As you're not part of the set up, it doesn't matter.

FWIW, my DH knows someone socially who has a polygamous set up. The couple are in their 70s and one has a partner that they spend a few days a week/several weekends a year with. It works for them, and they're all happy with the situation.

sigh and whatever. Not my circus, not my monkeys

It does seem like they are having an affair, but why do you think it would be exposed?
How does the husband know his wife and best friend are hanging out every week?

I think the likely scenario is he does know but it doesn’t bother him as he doesn’t love her anyway, and he gets a home and child/family out of the situation. If they split up he would lose both the home and getting to see his child all the time. Maybe he is taking the child to the grandparents specifically to give his wife time to see her affair partner. It sounds like it’s a situation that works for them. Only thing I wonder is if the best friend’s wife knows/cares.

I only started the thread to see if my suspicions of the affair resuming are correct.

We can all guess and assume and give opinions but you've no way of knowing this for sure unless you actually catch them being intimate!

Tough on the wife of the friend, really. Not fair to her.
everyone else in this situation seems to have been given a choice.

You said yourself it's not your business. Stay out of it and get a life.

Well My husbands best friend and I are very close and v good friends and spend time just the two of us regularly but there has never been anything sexual. He is like a best friend /brother. My dh spends time with my sister when I am not about. Again good friends and nothing more to it. We are all secure in our relationships and fully trust each other and whilst I know there has been gossip in our small village I really couldn’t care less as it’s nobody else’s business other peoples opinion really doesn’t bother me.

Either way, your brother knows about it. So it’s either platonic or they’re having sex. Sex isn’t an affair if your brother is aware. It’s quite normal for people to have different views on monogamy. Maybe she takes the kids on Tuesdays so he can see ‘Brenda’.
I know a very happily married (20+ years) couple. She has a few ‘nights out’ a week. He doesn’t because it’s not his thing. Works for them.

(Only read your posts, OP, and about half the replies.) What seems most likely is that he knows the affair's re-started on a weekly basis but doesn't much care because [a] he wasn't in love with his wife to begin with and [b] she's provided him with a home and family.

No reason to suppose this won't carry on indefinitely, unless you think the visitor's wife will divorce him for cheating? I'm surprised you're not more worried about that, as the main couple look pretty stable with their divergent sex lives.

What has any of this to do with you really? Do not get involved. Get on with your own life.

Why didn't you just say it was your brother 🤦‍♀️ if you think his relationship is a marriage of convenience anyway he may already know that she is sleeping with someone else and there is a good chance he is too. There are probably a lot of couples who live together for financial reasons and to still parent their children but aren't actually in a "relationship".

It's your parent's business if he moves back, and it isn't your decision. Everything around that is irrelevant. You said parents insisted he move out before, so it is up to them to hold firm to the decision. Nothing for you to do right now.

It's a really difficult situation because it's going to be almost impossible to protect your parents if for any reason your brother decides he wants to move back in. Especially if he's very manipulative! I can understand why you are concerned and hoping that the weird marriage dynamics might make this more of a possibility. If it were me, I would try to ignore all the strange stuff in the marriage but be very careful to be vigilant for any contact he has with your parents. If it does come to pass, try to remind them of the reasons why they asked him to leave in the first place. You really don't want him to become a resident again, especially with a child because it will make it virtually impossible for your parents to "kick a grandchild out on the street", as your brother will put it. Ultimately though it is their house.

Well it sounds like you think if the marriage fails he would buggar off back to live with his/your grandparents and take advantage of their generosity which may impact on their welfare and yours it would seem..have I got that right?

So they should say no. What’s the problem? He is not going to break into the house and camp with them. You are far too involved

Mind your own business this is nothing to do with you. Either distance yourself or accept its their choice not yours.

If the husband is aware and happy, then nose out.

Unless you actually state how you'll be affected, there's nothing else to say.

Why does it matter whether there's infidelity or not? If the marriage is working for them, the grandparents are happy seeing plenty of the DGC, you're not going to rock the boat because splitting up the marriage might result in him moving back in with the GP... Stop investing so much time and thought into other people's lives.

Sone couples swing or have open relationships. He may not be bothered about the faithful side. I'd stay out of it and leave him alone.

I should think both spouses are promiscuous to be honest. Husband surely filling his boots elsewhere if he's unfazed about her doing it.