All very good advice, except for:
"But just ask him. you don’t have to be accusatory, just say that you know the two of them have met up recently and that you feel it’s odd that it wasn’t on the calendar like his meet up with other friends are."
Cheaters are, by definition, liars. If you ask a cheater for the truth, they are going to LIE.
And their lies can be so astounding and flagrant that the betrayed spouse can start to disbelieve their own senses. I remember a few years ago, an OP came on a forum I was reading and said that she came home early one day; walked into her house; her H came bounding agitatedly towards her, shirtless and his hair dishevelled and saying he been napping; she saw movement in the bedroom behind him and started walking towards it; and her stark-naked neighbour came rushing past her, clutching her clothes to her chest, and vanished out of the front door.
And when OP overcame her speechless flabbergasted shock and started questioning, he said he was sick of her jealousy and the neighbour just came over to ask him to look at her toe, which she had jammed in a door.
And OP was so confused that she actually came to the forum to ask if she was unreasonable to doubt him!
Cheaters lie lie lie lie and lie. They've become the enemy, it is completely POINTLESS to ask them to tell the truth. They won't because that is partly WHY they cheat: they like lying and their lies.
I have zero problem with OP checking out what her H is doing. Cheating is abuse on many levels: mental, emotional, physical (STIs can be deadly), and above all financial. Marriage is a social contract: you need to protect yourself from partners who go bad and who can blow up your life.
And to the idealists on this thread saying, well, if you're checking him out, your relationship is dead anyway because you don't trust him: there is a LOT at stake, and NO ONE - including you - is going to willy-nilly blow up a decades-long relationship on the basis of an unconfirmed hunch.
This is dumb advice that does only hog-ties the victims of cheaters.