This is an Outlook problem, not an MJML or even an HTML problem.
If you inspect the HTML for the email sent vs what Outlook sends on a reply you’ll see it has changed the code.
Here’s an excellent highlight of this bug https://www.hteumeuleu.com/2016/super-mail-forward-an-email-...
Though you are technically correct, I just don't think this is a constructive attitude. Half the job of making emails is working around (Outlook) bugs. This is just the annoying world email coding.
If MJML is content ignoring Outlook bugs, MJML just isn't a satisfying tool for email makers.
Sure, it'll get the job done for some marketing, but now the developer isn't learning skills that will make robust emails beyond that.
Making robust emails becomes a completely separate skillset and the developer who chose to invest time in MJML will have to start all over learninging the necessary skills if he needs to make email you can actually reply to.
I’ve worked in email for a decade, you do what you can with the tools you've been given. An email will never look perfect everywhere and while I agree with the general sentiment you’re talking about regarding folks learning proper email code rather than relying on tools, I also think the quality of emails sent is substantially higher now thanks to those tools.
Are you complaining that his boss is forcing him to implement something that his end users can use?
If so, why isn't his boss right?
Personally, I'd say that yeah, he's supposed to work for his end user, why would he do anything else?
For example, if the org uses desktop Outlook but < 1% of your audience does, is it worth optimising the email for that audience based on internal feedback?
If someone notices that something weird happens when an email is replied to or forwarded, is that something worth acting on based on the real world frequency of that action?