Should the suitor be accepted or rejected after an Istikhara followed by a bad dream, knowing that such dreams did not occur before?

This dream does not signify anything, and it could be a self--reverie or from Satan, and not necessarily from Allah. It is advisable to accept the suitor if he is of good character and religious commitment. If the marriage proceeds, it is a result of the Istikhara; and if it does not, then be content with Allah's decree. One can repeat the Istikhara and seek the advice of family.