There may or may not be a problem here.
Your neighbor's wife died, your wife has helped out, both in helping in household chores and as a friend that he feels comfortable talking to. That doesn't mean they are betraying you.
He bought a new truck and while you inspected it, looked it over, they shared a laugh. That happens.
She bought a swimsuit and headed to the pool. Okay, she wasn't honest as to where the pool was, but did she tell you which pool?
Her friend has a secret apartment and her husband doesn't know. Yeah, that's for those 2 to deal with. Don't jump to conclusions and figure your wife is doing the same.
She went to a party with the girls. So the neighbor man was there. Maybe he was invited too. Don't read extra things into it. Were there other men at the party or was it mostly the girls and he was the only man? Maybe the girls felt bad for him being alone and asked if he wanted to attend.
The bottom line is that it may all be innocent, nothing bad going on. You need to sit down and talk with her and stop creating ugly scenes in your mind.
Hope all goes well for you.