New players don't know how this game works because of course they don't: they're new and inexperienced. Thus most people fall on safe and standard assumptions, such as healers only healing. Changing that mindset is very difficult and takes a lot of time. Time being the key point here. Telling a sprout that holy spamming is good mitigation in a random dungeon won't change their mind no matter how logical you're being. The sprout has cleared all content up to that dungeon without holy spamming and telling him to suddenly change it up won't work.

However, if the sprout player hears that tip continuously for a week it might just do the trick. Just like teachers repeating the important information multiple times in a lecture, there needs to be a sustained effort to educate. So if the sprout you're trying to help is receptive, then good, but if not, just give the advice and hope the veterans after you give the same advice later on. There is no catch all way to get to somebody.

Personally I try to maximise my DPS while on healing, but that habit didn't just come out of nowhere. It was the non-stop healer dps discussions at the end if HW on here and reddit that finally convinced me to do dps. I just needed to see the arguments more. If a random in a dungeon told me to dps I wouldn't have done it. In another anecdote I remember a DPS advising me to use Cure III in a specific situation, but I've never done such a thing before and despite wiping a few times I never bothered to take his advice. Looking back the advice was very sound, but I've only heard it once, and so it took me months until I realized why he said what he said.

So I would just say: give useful advice often and politely, and don't push it too hard. If the other person doesn't listen or respond there's really nothing you can do to make it better. You don't know what their assumptions are, let alone how they got those assumptions. Just hope they take good advice in eventually.