"We don't do quality tests, you know, we don't have the time for those things"
The individual in question (given experience and role) must and should answer truthfully regarding the product. However, this does not mean that he or she is 100% knowledgeable of the product. He or she may have expertise in a certain aspect, but to say "We don't do quality tests" would mean that the individual represents the entire company, and throughout the product's development, no one ever, QA tested (which may even just be an intern checking a table). In other words, the individual doesn't have the authority to speak on behalf of the entire product. Even if he or she does, then does he or she demonstrate with a degree of evidence that the statement is more likely true than false? A personal statement =/= a scientifically derived conclusion that there is no QA tests at all.
"Your colleague X is stressed? Oh, he should come to work here, we are sooo relaxed!"
This has nothing to do with the product, it is what I classify as small talk. If it distracts the client from the product, then it has no value to the table. Although I can detect the sarcasm it all depends on the client. In other words, each word you say in front of a client is valuable. If you can't find a topic that is constructive, can you argue that the quoted would add to the conversation?
"Our suppliers are no longer providing us this component we use in your product"
This is all about phrasing and timing. If it is known that there will be procurement issues, then document it and assess its impact. If the next best input costs $1 it is not a 'big' issue. Note it and make sure it is checked for accuracy. If it is $100 more, then definitely it warrants attention and discussion in person. If a person is truly a consummate professional, he or she doesn't just identify problems, but offer realistic solutions as well. Otherwise the problem is moot, the value of expertise is offering solutions, why hire someone who can tell you whats wrong but is unable to fix it?
Your colleague is smart, but doesn't see the whole picture. Different skills and talents come to play when 'selling a product', the technical engineers and scientists developing the product and the social engineers and scientists communicating with external actors (in this case the client). Take a moment with your colleague and point out these potential perceptions of his/her commentary and ask them if their contributions would add or detract from the overall goal (to make a sale).
Don't make the mistake telling him/her that he/she is wrong, that will only aggravate him/her and sour your relationship. Instead, frame the situation as a conceptual question of perception and allow him/her to modify his/her behavior with feedback from fellow team mates.