First: this is ONLY advice and if something does happen it's not our fault. We are not responsible for any harm you may suffer after reading this webpage. We also assume you are fit and you do not suffer from weak bones.
The body is a lot more resilient than you think. The human body can withstand 50 psi (pounds per square inch) and that's if it's a sudden impact. However if it's sustained pressure, the body can withstand up to 400 psi if the weight is gradually increased. Because the human skull is in an arch form, it can withstand large amounts of pressure. Same reason why bridges are usually arches. As for the rib cage, same principle.
Not many husbands like having their wives walk all over them, but I’m glad you do. Never feel ashamed of your fetishes. Being walked on like a carpet runner is just another way of being dominated. It’s a very common fetish. To quote Rihanna’s famous song S&M, “sticks and stones may break my bones, but chains and whips excite me.” It’s all about the pain leading to pleasure. In fact, pain causes the release of endorphins, which increases our tolerance for pain and triggers feelings of joy and pleasure. Sex and exercise are potent initiators of endorphin release and that’s why we are frequently left feeling happy and satisfied.
To answer your question, yes, your wife can walk all over you, literally. Whenever you want some hot and sexy foreplay, it doesn’t matter how heavy she is, as long as her weight is being distributed evenly across her feet. So that means no high heel shoes.
Pressure is force per unit area. So as long as the pressure--that means her body weight--is distributed over some sort of surface area, like her feet, according to science it’ll be a-okay for her to step all over you. It's similar to lying on a bed of nails. Nails are sharp but when you lie over many nails, the pressure on each point is distributed so you don’t get skewered like a shish kebab. Another example, is walking on broken glass. That’s a lot of pressure you’re putting on these tiny shards of glass, but because your weight is distributed under your feet, you don’t cut yourself. ONLY when you twist your feet into the glass and apply force, that is when you’re in trouble.
So since your wife is 250lbs and let's say she has size 9 feet--the average size feet for a woman who is 6 feet tall--the surface area of both her feet will be 3.6 (width) X 10 (length) = 36 square inches. Multiple that number by 2 because she's got two feet it'll be 72 square inches. Now since pressure = Weight/Force divided by area. That is 250 divided by 72 = 3.47 psi. So his wife will be standing on top of him with 3.47 pounds per square inch which is well below the amount the body can withstand. According to the barefoot bully blog, a guy who is 195lbs is able to jump on his victims causing nothing more than a bruise, which is what they want anyways. Quite amazing actually.
WARNING: these are all theoretical calculations and you should take precautions. Gradually add weight. That way the body can readjust accordingly.
The Barefoot Bully's website contains tips and even photos of him using people’s faces as foot stools.
There are also other ways to ease your wife into treating you like a human carpet. You could have her try to walk on your back and see how that feels and slowly move her way up to your front side. If she’s worried about the weight, she can always brace herself on a piece of furniture until she feels comfortable walking all over you.
I commend her on her willingness to try this for you. Make sure you return the favour.