The notice period would have formed part of the initial offer / contract that he received so should have been raised at that point if it was a true concern. As others have said, 12 weeks is a nice pay off if terminated.
However, if the workload requirement / expectations from company have changed significantly or if they have failed to put inplace appropriate support then there is room for - eg company has not fulfilled its obligtion ( or role changed without agreement) and therefore notice period could be negotiated
in the past I had a job subject to 12 weeks notice period. It was great. Lined up new job with agreed start date, resigned, carried out handover to new guy for a few weeks then had several weeks on full pay "gardening leave" to chill before starting with new employer.