Yes, I can agree that most men love attention from women. At least I do. Of course I never acted on any attention I received but I still liked to know that women besides my W found me interesting.
Be careful though with what your friend is telling you. It sounds like she is a little bitter and sour on men. While I think it's OK to raise a red flag when you see a note like you saw it is by no means evidence that he is going to have an affair or is even considering an affair. It seems your friend is pretty set on the fact that this note means more than it is because of her views about ways to show appreciation.
Really it makes no difference whatsoever what this woman did. Whether it be the note that you found or if she started stripping in front of your H it makes no difference. The key is how HE reacts to it. If he took the note as a comliment and never has another thought about this woman then who cares what she did. Of course if he acts on it then there is a bigger issue.
For now, you have no choice but to believe what he told you. Let it rest and don't dwell on it. Now is not the time to start mistrusting him. I know it's hard if he had an A on you once already but you have to live for now. If he proves to be dishonest again you can deal with it at that time. But there isn't any use going forward in your life with suspicions and the anxiety involved with that. I don't think there's anything going on anyway. If he was having an A he probably wouldn't have had you around and had such a good time with you...he'd probably be too busy with the person he was cheating on you with. Plus, he'd probably be too nervous to have such a good time with you. He'd be worrying about you or the OW finding out. His actions are not consistant with somebody having an A.
Keep your cool and do your best to notice his actions, which contradict an A, and try not to obsess over this note.
A dream it's true
But I'd see it through
If I could be
Wasting my time with you
-Band:Phish Song:Waste