I see lots of potential here and she looks awesome btw, dont make him go straight for it though as that would be too easy and opportunities missed, it be more interesting if he at first reluctantly tries to avoid the situation as he doesnt want to betray his best friend, this leads the milf to have to be more seductive and daring each encounter, teasing, choice of clothes with a few planned malfunctions, yoga in front of the boys while they trying to play the computer, accidentally spilling a drink on his lap so she gets the chance to try "rub" it dry and get half his clothes off, etc plenty of ideas but its all to get him so worked up and ready to explode he cant resist, thus begins their sneaky relationship.
Got plenty more ideas and how the son gets involved secretly at first, ill stop there as that was quite a large suggestion lol but happy to give the rest of what I came up with.
Also got alot of good suggestions for your other story with the wife that just got it on with the gardener so shout me if you wanna hear it another time.