I have heard that with some dogs like pomeranians you can ruin the coat by shaving it- it will never grow back the same. Scientifically I have a hard time understanding why this is true- why wouldn't the balance of guard hairs and undercoat eventually go back to what they were if the skin is undamaged?? I dunno.
Poodles though have no such problem. However, sometimes people talk about temporary issues with coat using such language as "ruin." For example, you could theoretically "ruin" a puppy's coat by shaving them down at 4-9 months or so because you aren't going to get back the same puppy coat. Poodles only grow puppy coat for <1 year and then it is adult coat. Losing a puppy coat prematurely would be something I might mourn, and say I/the groomer "ruined" it.
Also, people who enjoy showing the dog in conformation might likewise state the coat was "ruined" by something (bad haircut, chewed on, pine sap, poor job of banding the long hair) as a way of stating their frustration that the dog will be uncompetitive/"not able to be shown" for what will feel like quite a few months while their show coat grows back to the right length. It is not permanently ruined, however.
Some people are quite fussy about their poodle's hair, because why not? Poodles have fun hair and some of us enjoy trying to make it as perfect as possible. We might say a shampoo or conditioner we don't like "ruined" the dogs hair because perhaps a certain level of softness or manageability was "lost." This is just poodle owner drama, haha. Leave us alone or give us sympathy, we want to rant about our momentary poodle hair difficulties!
Diet: AKC champion poodles with gorgeous hair have been fed everything from high quality kibble to very complicated raw or home cooked meals formulated by veterinary dietitians. Many dog owners love to get into hot discussions about diet. Lots of strong opinions in this category, but a balanced diet whether kibble, home cooked or raw is not going to ruin a poodles coat. Some dogs with allergies might be able to maximize their coat by eliminating things they are allergic to. My hope/expectation is that the majority of well bred poodles do not have allergies (is this naive of me?).
Occasionally a poodle will pop up with a health problem that affects the coat like SA (Sebaceous Adenitis, an inherited disorder) or alopecia. These are the only situations where I think it can be accurately said the coat is ruined. The dog however is not ruined, just as lovable as ever. Assuming the dog is being well fed and cared for, with appropriate veterinary care, then this isn't something the owner has caused. We could go into a big discussion here about buying from reputable breeders who know their lines and do all the health testing, but you already understand that.
Regarding too much heat ruining the coat, hair can be burnt. Assuming the skin has not been damaged, the burnt part can be cut off and the hair will regrow, so no permanent damage. I truly hope we are not burning our poodles' hair! I imagine it is like with our own hair, if we do a poor job of blow drying it over a period of time we will get dull, dry hair that tends to break. For show poodles, or poodles with owners who love to do their hair, weekly baths and blow drying could be temporarily damaging to the coat if they are using poor technique or products. This is an area where the right tool for the job is needed. Companion pets who are always groomed by professional groomers every 4-6 weeks are not going to have coat problems. Many people with companion pets learn to do their own grooming (basic or quite professional). It is nice for a poodle owner to have a clipper (Bravura seems to be the basic clipper of choice for many pet owners), scissors (curved and/or straight), nail clippers or a dremel, a hi vac dryer (a Shelandy for $70 is a basic entry level for a companion), a slicker brush, and a greyhound comb. However, you can work up to this. A new puppy owner should start with nail clippers, brush and comb, along with finding a good groomer. A good groomer will coach you along and make your between-professional-groom efforts more effective.
. And sometimes I feel it is almost rude to ask the price lol, I guess people believe right off the bat that you can’t afford it if you have to question it. I’ve always hated the assumption, it’s absolutely ridiculous considering the day and age we live in.
I absolutely adore most cuts on poodles. All magazine worthy! 
