In reply to CantClimbTom:
> For perspective the Midi isn't that high ... it's not Everest.
It's 3,842 meters, which is high enough to notice if you're not acclimatised. Of course it's not Everest, and there are plenty of places in the Alps which are higher (but not so accessible to tourists), so supplementary oxygen seems OTT. Nevertheless even a healthy person may feel a bit breathless after even mild exercise. Altitude does place an additional load on the heart, even without exercise.
https://www.heart.org/en/news/2021/09/09/travel-to-high-altitudes-could-be-...
The other consideration is that it is a fairly rapid ascent of more than 2800 metres in around half an hour, so there isn't much time to adjust to the altitude. If he needs to get down quickly, then even if he can jump the queue as a medical emergency there may be a delay until the next cable car arrives.
It seems obvious to say that he should seek his doctor's advice, although most doctors have no experience of altitude medicine and are likely to err on the side of caution. He will have to use his own judgement and it will very much depend on how fit and healthy he is, but the article I linked to indicates that there is some risk. It would probably be a good idea to see how he gets on at lower altitudes before trying the Midi.
A possible alternative might be the Brevent lift which goes to only 2525 metres. It has good views of Mt Blanc, but not as close up as the Midi. Someone suggested the Montenvers train, which is even lower at 1913 metres and might be a good place to start, but he won't see Mt Blanc from there.