I think the majority of men who access pornography on a computer which their wife uses, will be careful to cover their tracks, and monitor their mails and browsing history very carefully.
The fact that these messages are effectively in 'plain sight' with no attempts either to open or respond to them does tend to infer that they are simply random spam, and this gentleman has no interest in pursuing the lurid offers he is receiving.
I would suggest that his wife finds a subtle way of letting him know that she has seen the material, and he should take steps to get it stopped.
If she is savvy enough, and has his contact info, she could always block the mails herself - he will probably think the spam has stopped because of lack of response, and things can return to normal.
Hope this helps.