I strongly disagree.
From builders and sellers point of view, there is no excuse to treat customer who paid a great deal of $ for your product in such a manner, even less excuse is having a bad day.
If you ever built a product in batches or 10-20+pcs of identical or very similar product (and I believe you did) then you exactly know what are the week spots of that product and what situations can happen during use of the product.
When someone calls me and say “hey this happened” I know exactly what he did that lead to failure.
Also as a builder that wants to sale his products you have to take in consideration that not everyone that will use your product is familiar with it, or that they will read manual or that they will care about the product at all, you have to predict as best you can all the things that may affect your product in negative way.
Someone may insert battery reversed, ok, you can either make reverse polarity protection or not but you know what will burn if this situation happens, or someone may put 2 small batteries because hey, years before they had flashlight that used 2 x CR123 batteries so they thought it will shine more if they put 2 batteries insteda of 1 18650 and the working voltage of the product is up to 4.5V. Either you will make product more expensive by designing it to be able to handle this situation or something will burn and you will know what was done to it.

Either way, buyer offered to pay just to diagnose the problem and give their opinion, maybe it is fixable, maybe its not, it is your product and you know that even without having it in your hands.

If the product is potted and there is no way to get to vital components without causing irreversible destruction then that’s what you need to explain to customer, and not act like customer is stupid and does not understand what 30-day warranty means.

When someone calls me and says “I have Acebeam…” with this or that problem, first thing I explain is that Acebeam is heavily using glue to prevent opening their lights, I can try to open it but that will probably cause some damage to the product (I always try to prevent this but I make sure that customer knows it may happen) on top of that if I open it and find the problem, it may still not be fixale (or I will not be able to fix it because I do not know everithing, im not EE).
After customer knows all this it is his choice if he will accept my terms or just keep the faulty product as a paper weight.

All in all, if you don’t want and dont know to deal with customers with problems (because sooner or later you will have unhappy customer) than get a job that does not involve interaction with people.