Oh, Hell NO chemicals.  

I get my dryer ducts and HVAC ducts cleaned every two years. 

The company I have hired for my home and recommend to my clients does not use chemicals.  They use a high-powered swirling brush.  They take before and after photos showing the vents are cleaned. 

Best to talk to the company who did the work.  However, I am not a fan of ANY chemicals in the air!  We are already surrounded by enough ... just too many toxins ...

An anti-fungal is made with chemicals. Homeowner should have asked exactly what was in the formula to avoid any adverse reactions.

You don't use a fresh air system for any other purpose. Imagine we hooked up a room freshener to it? You would choke on the air while smelling away! The way to treat vents etc., is soap and water WITH an upgraded, catch-all A/C filter after that. It catches, virus, dust, allergens, microbes, smoke plus several more. Salty people in the business know these things

I have friends/clients who get their ductwork cleaned. I know some Airbnb people did it often during the pandemic. I haven't checked into the chemicals nor have I heard about any adverse reactions.

No duct cleaning and no way with some chemical based product.  We did have them insulated but there was no smell or chemicals involved in that and it cut down our cooling bill by quite a bit. We change our filters often!

Duct cleaning YES big yes. I've had mine cleaned a few times over the years.

MAKE SURE THEY INCLUDE DRYER VENT!

In 2014 I had dryer running, and started to leave house to run an errand. I walked past and smelled something burning. Nothing on except the dryer. I opened dryer door and SMOKE.
The dryer vent had accumulated enough lint and dog hair over just a few years to clog the vent and it came this close to starting a fire.

I'm a religious lint-clearer but it never gets 'it all' esp with critters.

I was surprised as the ducts had been cleaned like 3x since we built the house. Found out that without specifically requesting dryer vent cleaning, they don't include it. I never noticed what they did and did not clean so I assumed they did the dryer vent too. Lesson learned and I'm about to have it done again this month.

I have no idea about antifungal spraying . . . I would not want that.

Cleaning - yet. Chemicals - no!

Homeowner needs to talk to the company, not your drama to deal with, maybe guide them. DO NOT EXCEED YOUR LICENSE!

I'm 100% with Lynnea Miller 

This is an interesting question as I don't have an HVAC, and so glad I don't have to think about how to clean the ducts.  But I do have to clean window screens a lot since they're always open.

I had mine cleaned when I replaced HVAC this year, no smell, just clean.  I also vacuum them as far as the hose will reach regularly and use a GOOD filter and change it frequently (every 3 weeks)

No chemicals for me, thank-you! I have had my dryer duct regularly cleaned but have never had my HVAC ducts done.

I've had them cleaned a few times & only once had them sanitized. No adverse smells & I didn't have to leave the house. Why didn't this guy ask when they were their doing it?

we had cleaning, insulation added and even a vent added where we felt we needed an extra when we replaced our 20 year old unit this past summer.  I was expecting lower bills, but inflation in energy costs saw that didn't happen.

Recommend cleaning every two years.  You may direct your customer to the following associations for further assistance: 

(National Air Duct Cleaners Association)

(Indoor Air Quality Association)

Never felt the need to spray poison in the ducts.

I may have a suspicious nature but, if it's not broke why "fix" it? If I call someone to fix our heater and they say the ducts should be cleaned, that is a red flag to me. How would they know that from fixing the heater?

Was the company that fixed the heater the same one that cleaned the ducts? If so, were they licensed and insured for duct cleaning? 

Did they actually clean the ducts or just spray the anti-fungal and consider that cleaning? Your friend needs to tell the company he wants an MSDS for the chemical that was used and it's unfortunate that he did not ask upfront what they were using and what it contains.

Suggest that he look up the company online for any complaints and also check BBB. I found the info below on the EPA site and he should read it paying special attention to the section on chemical treatment (Page 14 I think).

EPA booklet on duct cleaning

I've never had my cleaned or recommended clients to have theirs cleaned.  It's not a problem until it's a problem.

Not a fan of chemicals. I'd leave the fan running until the smell is gone.

No idea what chemical they used. Did they think they had mold?

I've had a company that uses and air compressor and wire brush like a chimney sweeper. Dust, hair, stuff can get in there. I wouldn't want any chemical added.

Our climate is very dry in Colorado so there is no moisture for stuff to grow in a heating duct. Maybe cactus?  Dryer vent has moisture. 
I change the furnace filter often to remove dirt and dust.

Good reminder to review MSDS info for any chemicals used by a contractor or homeowner for cleaning, garden, lawn care, pesticides, painting, automotive, hobbies, etc.  Store chemicals safely.

You can buy a whole-house in-duct Ultra-Violet C (UVC) germicidal air cleaner for $100 on Amazon.  No chemicals needed, like a bug zapper...

I would absolutely want to know if they spray a chemical into the ducts!

I've never heard of, or even knew of anyone having the chemical treatment.

I can't recall anyone spraying chemicals with duct cleaning.