Not sure why you think they don't understand photography, using a tele wide open for portraits is a fairly common technique (something like a 200mm for example) as it gives a narrow FOV and shallow depth of field, isolating the subject from the background. Getting closer with a shorter lens and therefore a wider FOV is another approach if you want to put the subject in context more but there's nothing wrong with using a tele. It just depends what you're trying to achieve visually.
Also, although you're right that small sensor cameras usually struggle at higher ISO's but perform perfectly well in bright lighting conditions it's also the case that larger sensor cameras (like DSLRs) also give more control over depth of field which is particularly useful for portraits.
Of course whether it's worth the investment in gear and money in order to take what may or may not be mediocre pictures of their girlfriend in the park is another question, but there are perfectly sound reasons for using a DSLR + tele for portraits from a technical point of view.