If you have a wife who seems to want to fuck a strange cock an you're anxious to see it happen, by all means go for it.
My suggestion - ask the old guy into the house for a drink some evening - tell you wife that you think the old guy needs a thrill in his life and to wear something sexy.
Tell your wife your thoughts - tell her you would enjoy watching her fuck the old guy or give him a blow job or at the very least a hand-job. Tell her when he comes over for a drink that "anything goes".
When the old guy does come over - chit-chat for a while and then leave the guy and your wife alone for a couple of minutes just to see what happens.
If you peek into the room and they're fondling each other you've got what you wanted. Just let it all develop.
Of course - personally I love to be in the room when my wife fucks - but if you'd rather them go alone to your bedroom, that's your choice.
In any event, this could work out to be a great weekly fucking event for all 3 of you folks. And the bonus is you get to fuck your wife's pussy with second-hand cum filling her twat - yum yum.
Sounds like you have a great opportunity!!!!
FUCK MY WIFE!