OK, to clarify, as they are owners of separate properties then there is no issue with them both being on the committee.
As for member A nominating member B to represent them on the committee, that is up to the committee to decide (and it has agreed to this).
The problem is whether the two owners have “failed to exercise due care and diligence” or if the Tribunal might feel they had seriously misled the owners in the building when they stood for election knowing one of them was going to be absent for most of the year.
The simplest way for you to resolve this may be to request mediation at Fair Trading (which is free), asking the chair to hold a vote to remove the approval of the substitution with a clear message that if they fail to do so you will seek the removal of the absent member AND their partner from the committee under section 238.
I’m still not sure how one member of this couple can go on holiday for a year and leave the other one to attend committee meetings, but there you go.
And just to be clear, you are not going to get definitive legal advice on this forum. We can only point you in the direction of the law and usual practises.
This is very much one of those cases of “suck it and see” when it comes to strata law and the first thing you need to do, if you wish to pursue this, is request mediation at Fair Trading (which you would need to do if you were going to take this to the Tribunal in any case).