Okay, let's start over. Let's assume that in 'a friend with heart disease', 'with heart disease' is the equivalent, function-wise, to a relative clause with only the finite (either present or past) forms of 'has' and 'had' (I say only the finite because "have" cannot used in the progressive
here).
Now, just because 'with heart disease' is functionally the equivalent (it also modifies/defines/describes the noun 'a friend') of a relative clause does not mean that it too enters into a temporally meaningful relationship with the context (particularly the other verbs) taken as a whole (indeed, a preposition cannot inflect and therefore
cannot be in any temporal relationship with the overall context in the same way that a meaningfully-inflected relative clause would obviously [have to] be); put simply, we have to look elsewhere than 'with heart disease' to know 'what the "time" is'.
Where should we look then? At the surrounding context, obviously enough (inside which the modified noun phrase is simply a "character" in a text, which here is a "past" story).
Given all the above, 'a friend with heart disease kept insisting that...doctor.' simply means/can be paraphrased as 'a friend who (still) had heart disease' (and in a list of characters this friend could be chracterized as 'a friend with=who (still) HAS heart disease'). No surprises there!
And even though this friend has had (=had had!) medical advice which has saved (=had saved!) his life, we cannot assume from the context that his ailment was totally cured (and hence "past within an already past context") -
to be totally sure, there'd need to be an explicit use of a more complex form than 'with': 'But a friend WHO HAD HAD (='who had used to suffer from' - different from 'who had suffered from'!
) heart disease kept insisting that Benson should seek further medical advice and visit his doctor. The friend claimed the guy had saved his life (and cured him somehow).'
That is a very long answer, perhaps I should just abruptly (tough sage advice-meister!) say, whilst winking, that 'Past perfect is used for a reason, you know!', or something similarly short and sweet.
Edited-in bit: I see SJ beat me to it with some sage advice! (Actually, I noticed he'd posted something, because I keep two browsers open to save scrolling down to read what's been written, and clicked forward and back again just the once to see if anyone's name in the "answer listings" had replaced Seiichi's, but I resisted the temptation to actually read what SJ had written until I had finished writing my answer above...and even if I had read his post midway through mine, I feel it would've made little difference to what I was going to write and wanted to say
).
).
I really should consider getting more of that thing known as "sleep"... 
