What do I tell a friend who is worried about his wife using a dildo?

I have a friend who is religious and traditional, but with a sense of humor. I’ve jokingly told him and his wife that they should get some sex toys and have a little fun. Recently, they decided to purchase a dildo.

Here’s the problem: my friend fears his wife prefers the dildo. I believe he is genuinely threatened by the dildo (which is slightly more enhanced in length and girth than what his wife is used to). From the start, he had forbidden her from using the dildo when he wasn’t involved. From what I gather, they had fun with it at first, until a certain incident: his wife was using the dildo while giving her husband oral sex. At some point, he decided he was “ready” and wanted to proceed to vaginal sex, but she stopped him and said she wanted to enjoy the dildo for a few more minutes. This deeply troubled him. When he tried to talk to her about that particular incident, she kept avoiding the subject.

Now he is very worried that she prefers something bigger than what he has to offer. I tried explaining the silliness of him being jealous of an inanimate object, but he countered that, jealousy aside, he was worried the dildo would “stretch” his wife out, making sex less enjoyable for him in the future. Considering that she’s had several kids, I doubt this could happen.

Is it all in my friend’s head? Is it really just penis envy? What solid and factual information could I provide him to make him feel better about the whole thing? Are humans capable of being jealous of a piece of plastic? My personal belief is that inanimate objects are the only way a person can “cheat” on his partner without arousing genuine jealousy. After all, a woman doesn’t get jealous of her husband’s hand when he masturbates, right? Can a dildo ruin a marriage?