Is he exposed to smoke there? That will definitely cause an asthma flare. Or maybe pets, or trees/grasses that aren't at your home? Insist she give him his meds, or you're "afraid he won't be able to visit". I've heard of people who've had to get a court order to have one parent who "didn't believe in meds" agree to give them, or risk losing visitation.

He is not exposed to smoke there. It may be the dust from it being an older home, it may be some plants in the home, or it may be dander from a cat she had years ago. Whatever it is it causes his asthma to flare up.

She does give him his meds when he is there, but she just complains about it.

At least it's not cigarette smoke --
And it's good that she's giving him his meds --

But there's SOMETHING there that's triggering your son's asthma. How bad is it? Asthma can be life-threatening, or (IMO) it can be a nuisance. How bad is SS's asthma when he's there? If his asthma is serious, then you really need to either find out what's causing it and/or not allow him to stay over until you get it fixed. I assume you've had the full workup of allergy tests? What does his doctor say about allowing him to stay over and finding the source?

Either way, I wouldn't sugar-coat it. I mean, what's the point?
If she's not smoking and she is giving him his meds (even reluctantly), then it's not her 'fault', and she shouldn't take offense to your following his doctor's advice.

We have done the allergy tests and no allergies. So we are not sure what is triggering the asthma at her house. He gets home from there and the wheezing begins. It is not life-threatening or we would have stopped the sleepovers.

What gets me is that before I entered the picture my dh was working midnights and the kids were sleeping at mil's every night and there was no regular problem. But ever since he stopped sleeping there so much and only goes for occasional visits his breathing gets bad there. It is almost like he had an immunity built up before to whatever it is thats bothering him now.

I know it is not her "fault", but my dh seems to think it would hurt her feelings to tell her that it is her house that makes him wheezy.

I know everyone is tight for money, but maybe DH could offer to pay for carpet cleaning, etc. I had my carpets cleaned and it did help get rid of cat dander. Not perfect, but helped.

She does not have carpet. All hardwood floors and tile floors.

What if you address it totally 'innocently'? "MIL - it's so odd - SS was doing so well with his breathing, but we noticed when he came home he seemed to have troubles again. We know you would have given him his medicine, so we are wondering if maybe you have some different trees than we do that could cause this? What do you think it couldbe?" If you don't make it a confrontation, but rather ask for her sage wisdom it might be better received?

If you're sure she's giving him his meds, is there any chance she's giving him herbs in addition to his meds? She sounds very stubborn about the whole "meds are bad / herbs are good" thing, and some herbal/'natural' remedies can definitely interfere with medications.

Could it be food related? Some special health food or herb she's taken to cooking with? Changing from regular milk to soy milk or vice versa?

Any new houseplants? Potpourri? Scented candles or Glade air freshener? Using different cleaning products? Does she ever use Febreze/Oust/CarpetFresh type products (they can trigger asthma)?

I think you need to find out what's triggering his asthma, and it doesn't have to be personal - something at her house is bad for his asthma, and you need to figure out what it is so that he can avoid it. It's not personal because whatever the trigger is, he'll need to be careful of it anywhere it might be. And it'll make his visits more fun - struggling to breathe takes the fun out of any situation.

My bet is that he's just plain developed an allergy to something over there for which they don't test. Allergy testing doesn't test for every single substance in the universe-- it typically tests only for the most commen allergens. It's possible to be allergic to something for which there is no test. For example, I very definitely became allergic to something in Secret deodorant that is not present in other deodorants. It makes me itch on contact. But there is no test for, "Mysterious Thing in Secret." I just know it's an allergy because it's prompting an allergic reaction, and I have other allergies. And this is an allergy that developed-- I'd used Secret for years before it started making me itchy.

If I were you, I would stop the overnights, much as they both might enjoy them. Some allergies can be escalating, so that each exposure results in a more intense and prolonged reaction. If your stepson is genuinely having an allergic reaction, I think it's possible that his asthma attacks could get stronger to the point at which they are life-threatening. And I would pretty much say that to her, unless she's so opposed to traditional medicine that she wouldn't regard it as a valid reason.

This is kinda gross but I've read that asthma in children is especially triggered (or exacerbated) in places where there are a lot of roaches. Does she live in such a place? (Not to imply she's a bad housekeeper, but sometimes older houses, especially in certain climates, can have them no matter what you do....)