welcome to magic health care.
from your post, it appears that your brother in law has a new chest discomfort
which was not there till now (not present in post surgery phase, even on walking). if that is correct, then you must see your cardiologist at earliest , because this may be a sign of partial/ complete closure of one of the grafts (the additional blood vessels used to bridge the previously blocked vessels) which may necessitate detailed work-up. Another possibility can be a new blockade in native vessel.
but, the kind of description of symptoms which you have given, is more in favour of a cardiac discomfort, rather than non cardiac, so you must see your cardiologist.
will please to answer, if there is a further query