The question doesn't contain enough details for the exact answer. I had an experience with substituted N-methylpiperazines which formed a N-methylpiperazine borane complex after reduction of the other part of the molecule with sodium borohydride. Some trialkylamine boranes are extremely stable to acidic hydrolysis, even with 1 M HCl (see R. O. Hutchins et al. Organic Preparations and Procedures Int. 1984, 16(5), pp. 335-372). The problem was solved by using water formaldehyde solution which reacted with the amine borane complex to form methanol.

I think B(OH)3 might coordinated with N-atom ,B(OH)3 might formed after quenching with water so its better to use 4 M HCl in dioxane ,same problem was encountered by my team.

Procedure::

Take your compound in in any nonpolar solvents ,i recommend MTBE then cool to 0-5 oC then add 4 M HCl in Dioxane till pH 1. stirr for 1 h. you will end up with N.HCl salt. which can be filtered. B(OH)3 will become as BCl3 which is a gas it may not add to N atom in acidic condition .