It appears that the initial access is not extended lingually (L) enough (Fig.5 occlusal view). The lingual canal is still non-negotiable when the access is enlarged (Fig.6 mirror occlusal view). B: buccal. The same effort is exerted for his wife.
Missing Lingual Canal
Xin Wei, DDS, PhD, MS 1st edition 11/23/2014, last revision 11/23/2014