Thanks for the opportunity to respond to the issue your friend is facing.
Its somewhat similar to what a co-practitioner was going through some years ago. I heard about on the grapevine that he felt that he should 'just observe' the sensations that were manifesting due to a chronic ailment. And it was getting to the point where it was debilitating. When the area teacher found out, he was instructed to seek medical treatment.
The particular practice that we are mainly involved in, vedananupassana: (observation of sensation) is to develop moment-to-moment awareness of the anicca characteristic of sensation while remaining equanimous to sensation. i contend that one cannot have sati if one is not equanimous - but that's another subject for another day. The sort of thing that my co-practitioner was gettng involved in was a subtle aversion towards his body which was manifesting as a desire to just observe the disease state no matter what it cost him physically or mentally.
U Ba Khin had a great saying in the discourse he gave in 1968 called the Essentials of Buddhadhamma where he was talking about how one should practice in daily life. He said that it wasn't necessary for one to maintain awareness of anicca all the time. He said 'one should work when you work and play while you play', meaning - reserve times of observing anicca to the set periods of meditation and the rest of one's time one should just get on and do all the things one needs to do in day-to-day life.
My personal opinion is that one should never ignore possible symptoms of illness or disease. Certainly, there maybe ample time to observe their anicca characteristic, but one should also ensure that one listens to one's body and acts accordingly - ensuring our health and longevity - which in turn ensures an ongoing opportunity to practice Dhamma.
Also remember,only some vedana are produced by sankhara.
“No lists of things to be done. The day providential to itself. The hour. There is no later. This is later. All things of grace and beauty such that one holds them to one's heart have a common provenance in pain. Their birth in grief and ashes.”
- Cormac McCarthy, The Road
Learn this from the waters:
in mountain clefts and chasms,
loud gush the streamlets,
but great rivers flow silently.
- Sutta Nipata 3.725Compassionate Hands Foundation
(Buddhist aid in Myanmar) • Buddhist Global Relief